Volume 3 solar thermal systems components and applications 3 15 – solar assisted heat pumps

Volume 3 solar thermal systems components and applications 3 15 – solar assisted heat pumps Volume 3 solar thermal systems components and applications 3 15 – solar assisted heat pumps Volume 3 solar thermal systems components and applications 3 15 – solar assisted heat pumps Volume 3 solar thermal systems components and applications 3 15 – solar assisted heat pumps Volume 3 solar thermal systems components and applications 3 15 – solar assisted heat pumps

DA Chwieduk, Warsaw University of Technology, Warsaw, Poland

© 2012 Elsevier Ltd All rights reserved

3.15.3.1 Classification, Configurations, and Functions

3.15.4.2 Classification and Evaluation of Seasonal Ground Storage

3.15.4.3 Heat and Mass Transfer in the Ground Store, General Consideration

References

3.15.1 Introduction to the Concept of Solar-Assisted Heat Pumps

There are some limitations on the use of solar radiation for heating purposes, mainly because of its stochastic and intermittent character There are changes in solar radiation availability in the long term, that is, over a year, and also in the short term, that is, over

a day When solar energy is used for space heating, the time and peak values of available solar radiation are quite opposite to the time and peak values of space heating demand This is especially true in high-latitude countries, where in winter the solar radiation level is low and the duration of solar irradiation is short, and consequently there is a cold climate and long heating season with high heating demands

In many high-latitude regions, heating of buildings is a major component of the total energy used in the building sector Usually

‘traditional’ solar active heating systems alone cannot provide all the heating needs There are different options to solve this problem One of them is to couple a solar heating system with a heat pump in one combined heating system, as can take place in a small- or medium-scale application, that is, in single-family houses and multifamily or public buildings, respectively This type of a heating system is called a solar-assisted heat pump (SAHP) system Another option is to use the seasonal solar energy storage in the form of sensible heat of a large storage volume Usually the temperature of the heat stored is too low to be used directly for heating The low-temperature heat stored can be converted into higher temperature heat by applying a heat pump In this way also a heat pump is incorporated into the heating system Such heating systems can be used for medium- or large-scale applications, that is, in multifamily

or public buildings, or blocks of buildings (i.e., in communes and small city districts), respectively In the case of medium-scale applications, the heating system is termed a solar-assisted heat pump system with seasonal storage (SAHPSS) system, and a common example is a solar-assisted heat pump system with ground storage (SAHPGS) In the case of large-scale applications, such a system is called a central solar heating plant with seasonal storage (CSHPSS) There are a variety of underground thermal energy storage (UTES) system configurations and modes of operation Solar collectors and a heat pump are the major system components

This chapter describes the concept, classification, and operation of SAHP systems, including systems with seasonal storage For better understanding of the idea of SAHP system operation and application, the fundamentals of heat pumps are initially presented Heat pumps applied in SAHP systems are vapor compression heat pumps, and this type of heat pumps are described and analyzed

3.15.2 Heat Pump Fundamentals

3.15.2.1 Principles of Heat Pump Operation

Heat pumps, refrigerators, and heat engines are heat machines Their operation is based on thermodynamic processes that are governed by the first and second laws of thermodynamics [1, 2] Heat pumps and refrigerators operate in reversed cycles compared with heat engines The processes and energy flows are in the opposite direction to those in the power cycles of heat engines, as presented schematically in Figure 1

Comprehensive Renewable Energy, Volume 3 doi:10.1016/B978-0-08-087872-0.00321-8 495

Power cycle Reverse cycle

Figure 1 Concept of power cycle for a heat engine and reverse cycle for a refrigerator or a heat pump

A heat engine’s function is to generate work using a heat source In a heat engine, the high-temperature T2 heat source is used giving the heat input Q2 to get the energy output in the form of work W However, there is an amount of heat Q1 at lower temperature T1 that must be removed to the heat sink to fulfill the first law of thermodynamics The first law of thermodynamics is the conservation of energy law, and can be written (see Figure 1) as follows:

Unlike a heat engine, a heat pump’s function is to lift a certain quantity of heat Q1 from a heat source at a lower temperature level T1

to a heat sink at higher temperature T2 However, to fulfill the first law of thermodynamics, the upgrading of heat must be done by the work W that is supplied to the machine Following this and Figure 1, it can be seen that the amount of heat Q2 to the heat sink is the sum of heat Q1 extracted from the heat source and the amount of work W required in the process, and the first law of thermodynamics, described by eqn [1], is accomplished

As it has been already mentioned, a heat pump is used to supply heat Q2 at high temperature T2 Conversely, a refrigerator is used to extract heat Q1 at low temperature T1 It means that a refrigerator’s function is to cool down a given heat source extracting

a certain quantity of heat Q1 from this source (at lower temperature T1) In a refrigerator, as in a heat pump, to fulfill the first law

of thermodynamics, to extract the heat from the heat source the work W must be supplied to drive the cycle and heat must

be removed at a higher temperature In practice, the functions of both heat pump and refrigerator can be combined in one machine, when both heating and cooling are required simultaneously For example, such situations can be found at a sport center where some chillers are used to cool an ice skating area and at the same time they also provide heat for hot water for swimming pools, operating like a heat pump

The efficiency of a heat engine, a refrigerator, and a heat pump is defined using the first law of thermodynamics In the case of a heat pump and refrigerator, efficiency is measured by the coefficient of performance (COP), which is the ratio of the energy that is used for heating (at the heat sink) or for cooling (at the heat source) to the work that has to be supplied to drive the cycle The efficiency of a heat engine, a refrigerator, and a heat pump can be expressed by eqns [2a], [2b], and [2c], respectively, in the following way:

According to the second law of thermodynamics, heat cannot flow from a lower to a higher temperature without the expenditure

of energy This law for the reversible cycle can be expressed by the following equation:

3.15.2.2 Thermodynamic Cycles

In principle, to achieve the reversible cycle in a heat pump, a condensable fluid must realize a reversed Carnot cycle This allows heat

to be input and output at a constant temperature by means of boiling and condensation This meets the requirement that all heat transfer to and from the system must be reversible The Carnot cycle [1–4] for a heat pump in a temperature–entropy diagram is shown in Figure 2 on the left and the main components of a heat pump based on the Carnot cycle are shown on the right The following processes presented and numbered in Figure 2 take place:

1–2 Isentropic compression Two-phase liquid–gas of the working fluid from the evaporator flows into the compressor and is compressed to the required level of pressure and temperature

2–3 Constant pressure and temperature heat rejection – condensation Vapor of the working fluid at high pressure and temperature flows from the compressor to the condenser At constant pressure and temperature, the working fluid condenses giving up the heat (latent heat – condensation heat) to the sink, for example, space heating medium that provides the heat required by the space heating demand

3–4 Isentropic expansion Liquid of the working fluid from the condenser flows into the ideal expansion device (at high pressure and temperature) and is expanded to the required level of pressure and temperature to close the reversible cycle

4–1 Constant pressure heat absorption (evaporation) The two-phase liquid–gas working fluid at low pressure and temperature flows from the expansion valve to the evaporator At constant pressure and temperature, the working fluid evaporates, because

of the existing temperature gradient between working fluid and the low-temperature heat source Then the process repeats

Equation [6] is true for only reversible adiabatic processes and then S = const Analyzing Figure 2 it can be seen that two processes are adiabatic: 1–2 compression and 3–4 expansion Because of the equity of entropy differences, S4 – S1 = S3 – S2, the efficiency of the Carnot cycle for a heat engine (eqn [2]) is as follows:

The same can be written for the efficiency (i.e., COP) of a refrigerator and a heat pump operating in reversible adiabatic processes

1–2 and 3–4 (Figure 2), and according to eqns [2b] and [2c] respectively, it is as follows:

The ideal reversed Carnot cycle is realized between heat source and heat sink that have constant temperature, that is, T1 = const and

T2 = const The transition process between these two states (lines) is adiabatic, and can be written in the following way:

The main components of a heat pump specified below are responsible for processes presented and numbered in Figure 2: 1

The following processes presented and numbered in Figure 3 take place:

1′–2′ Isentropic compression Saturated dry vapor (of the working fluid) at low pressure flows from the evaporator into the compressor and is compressed to the required level of pressure and temperature in the superheating vapor region

Figure 3 The vapor compression cycle for a heat pump presented in a temperature–entropy diagram

4′–1′ Isobaric and isothermal evaporation Two-phase mixture of the working fluid at low pressure and temperature from the throttle flows through the evaporator At constant pressure and temperature, the working fluid evaporates, taking heat from the low-temperature heat source There is no pressure drop in the evaporator and connecting piping

Comparing the evaporation process of this cycle and the Carnot cycle, it is evident that now the end of the evaporation process is exactly on the saturation line (point 1′) It must be underlined that in reality stopping evaporation at just the right dryness fraction,

as it is in the Carnot cycle, is very difficult In addition, real compressors might be damaged because of compressing two-phase mixtures (saturated liquid and vapor) to a saturated vapor state

In practice, to ensure that the evaporation is really completed and there is only one-phase fluid (saturated vapor) at the compressor inlet, a small amount of superheat (typically a few degrees) is transferred to the vapor just after leaving the evaporator This is shown in Figure 4 in the T–S diagram where state 1″ (beginning of ‘suction’) is slightly superheated

Using a throttle in the place of an ideal expansion device means that the expansion process is not reversible There is saturated liquid leaving the condenser which flows into the throttle However, the reduction in pressure in the throttle causes some of the liquid to boil and a two-phase mixture is formed As a result, the temperature of the working fluid drops The whole process is isenthalpic, because the enthalpy of the stream of fluid is the same before and after the throttle However, two phase mixture leaving the throttle contains liquid with lower enthalpy than the fluid before throttle and vapor with higher enthalpy than the fluid before throttle This means that the working fluid enters the evaporator at point 4′ not 4 and the entropy of expansion process in the throttle increases and the process is irreversible

Entropy difference at the heat source is not equal to that of the sink, the former being bigger than the latter At such cycle, the work input W to the device must be bigger than that for the Carnot cycle (the rectangular area matched in Figure 2) As a consequence, the vapor compression COP is reduced in comparison with a Carnot cycle working between the same temperature limits, and can be written as follows:

The vapor compression cycle is very often presented as the ln(p)–h diagram An example of such diagram for the vapor compression cycle is shown in Figure 5

point 2‴, which represents the state of the desuperheated vapor after the compression, which in reality is not an isentropic process The points in Figure 5 represent the following processes:

Figure 4 T–S diagram of the ideal vapor compression with the beginning of ‘suction’ slightly superheated

win = h1″ – h2‴ = –h1″2‴ – adiabatic compression process (q = 0)

It is very convenient to use the ln(p)–h diagram to determine the COP of a heat pump, because the COPs are simple ratios of length (enthalpies) in the ln(p)–h diagram, as presented in Figure 5 A COP of a refrigerator and a heat pump (referring to eqns [2b] and [2c], respectively) can be expressed as a function of enthalpy The enthalpies of different refrigerants as functions of pressure and temperature can be found in the literature (e.g., in tables and charts [5]) Thus, the COP of a refrigerator and a heat pump, using the diagram in Figure 5, can be expressed as follows:

3 is moved into the liquid region, to the left) This makes the input work W to drive the machine bigger than for a Carnot cycle, as indicated by the increased area enclosed by the cycle in a T–S diagram, and as a consequence the COP is smaller It must be also underlined that in real heat exchangers there is a temperature difference between evaporating working fluid and a heat source (e.g., about 10 °C) and between condensing working fluid and a sink (e.g., 5 °C) It makes the temperature difference in a vapor compression heat pump between a heat source and a heat sink bigger than that in an ideal Carnot cycle heat pump, and makes the COP smaller Another reason for the reduction in COP is the nonideal compression process Compressors operate with a certain isentropic efficiency and in addition electric motors driving a compressor operate with an efficiency less than one Therefore, in practice, COP drops to 3–4

It should be mentioned that nowadays some compressor heat pumps can offer apart from the heating an additional function by being able to cool buildings There are two main different methods for cooling with a heat pump In the first method, a heat pump can operate in a reversible cycle, so in summer it operates like a refrigerator (the fundamentals have been described in the beginning of this chapter) In the second method, the so-called direct ‘natural cooling’ takes place It means that a heat pump is switched off, except for the control unit and the circulation pumps The ‘natural cooling’ is applied

in ground and underground water heat pumps when the brine of underground heat exchangers or the groundwater system absorbs the heat from the heating circuit (e.g., floor heating system) in a building and transfers it to the heat source medium

in the ground This method can also be considered as thermal regeneration of the ground heat source (cooled during heating season and heated during summer)

One of the most important issues of heat pumps is requirements for refrigerants Refrigerants should have low pressures (all subcritical) at heat exchange temperatures They must be nontoxic, not flammable, and nonpolluting, that is, environmentally safe Considering their influence on the environment, the following indexes are usually used:

• ODP – ozone depletion potential;

• GWP – global warming potential;

• TEWI – total equivalent warming impact (for the whole system)

It is also required that the refrigerant’s density is high for low-volume flow rate and thus a smaller compressor, piping diameter, and heat exchangers can be used In the past, the most popular refrigerant for domestic and light commercial heat pumps was R22 Nowadays, it has been replaced mainly by R 134a (a hydrofluorocarbon (HFC)) However, there are a number of new (and old) refrigerants such as other HFCs, propane and butane mixtures, ammonia, or carbon dioxide Another important issue is the selection of a heat source suitable for heating demands and the type of heat pump as described below

3.15.2.3 Classification of Heat Pumps

As has been described in previous paragraphs, the general principle of a heat pump operation is to extract heat from a low-temperature heat source and to transfer it to a heat sink at a higher temperature The useful energy output must be significantly greater than additional energy required to drive a heat pump to achieve a real reduction in primary energy use Heat pumps can use renewable energy or waste heat as a heat source Energy extracted from these sources is converted into useful heat in the low-temperature range This low-temperature heat can be applied with high efficiency, for example, for space heating and domestic hot water (DHW) [6]

A number of different classifications of heat pumps can be made; the main one is according to the form of energy that is used to drive them In this case, the following types are considered:

• mechanically driven, that is, compressor heat pumps;

• thermally driven, that is, sorption heat pumps

In this chapter, the mechanically driven compressor heat pumps are considered Among compressor heat pumps, we can list

• electrical heat pumps in which the compressor is driven by electricity (e.g., by an electric motor);

• heat pumps in which the compressor is driven by an internal combustion engine; these heat pumps operate with natural gas, diesel, or biofuel (rapeseed oil)

The heat pumps considered can be classified according to the type of the end user as follows:

• domestic heat pumps for small- and large-scale application used for space heating and DHW;

• domestic heat pumps for small- and large-scale application used for space heating, cooling, and DHW; these heat pumps can work depending on the season of the year (winter and summer) in heating or cooling mode, which means that a heat pump function can be reversed;

• light commercial heat pumps for different heating purposes (applied in offices, schools, hotels, hospitals, public buildings);

• heat pumps with dehumidification function (for swimming pools; for drying of vegetables, fruits, plants, etc.);

• large commercial and industrial heat pumps (for town districts and towns, industrial applications) These mainly use waste heat sources The sources of waste heat can be sewage, exhaust gases, or technological waste heat in air, water, and vapor

Classification can also be made according to the heat pump’s construction There are two fundamental options:

• compact or unitary heat pump – all the components are in one compact unit, there is a heat exchanger between a heat pump evaporator and a heat source, and between a heat pump condenser and a heat sink Usually the heat source is outside the building heated; however, when the waste heat is used it could be also inside a building;

• split heat pumps – the components are split (divided) usually into two units: one of them is located in a separate room or outside

a building Usually an evaporator or a condenser can be located directly at the heat source or at the heat sink, respectively

It means that evaporation and/or condensation take part directly at a heat source or at a heat sink When the evaporator is located directly at a heat source, such systems are also called direct expansion

The other way of classification can be done in accordance with the role of a heat pump in a heating system, and it is as follows:

• monovalent heat pump – it operates throughout the year in monovalent mode providing all heating requirements by itself;

• bivalent or hybrid heat pump – to provide all heating requirements it has to operate in conjunction with another heating device

or system

Heat pumps can also be classified according to the type of a heat source medium, which can be generally air, water, and brine (antifreeze mixture) They can use waste or renewable energy heat sources Renewable energy sources are

mainly used for domestic and light commercial (including swimming pools) applications The renewable energy sources are

3.15.2.4 Renewable Heat Sources

Generally, when heat pumps are considered for effective use, the following characteristic features of a heat source are taken into account [6]:

• good availability;

• coherency between the source and the user;

• high thermal capacity;

• constant in time and of relatively high temperature;

• natural energy equilibrium of the source (environment) and its physical characteristics are not affected by heat extraction;

• high purity (to avoid corrosion);

• no pollution, damage to environment, and other ecological issues;

• low cost of heat extraction

Renewable energy heat sources are described below and their characteristic features that have been just mentioned are analyzed briefly The simplified idea of utilizing different renewable energy sources for a heat pump at a single-family house is presented in Figure 6 Considering availability of heat sources ambient air is the best Unfortunately, it is not coherent with space heating demand When space heating demand is the highest, the air temperature is the lowest The temperature of ambient air is not constant in time and it can fluctuate very rapidly Ambient air (heat source) heat pumps operate usually with a COP of about 3 When the temperature of ambient air is just above 0 °C, then the problem with ice formation on the evaporator surface can occur This surface ice effect causes worse heat transfer conditions and together with the low ambient air temperature they result in low thermal performance of a heat pump, that is, low COP Nowadays, these possible problems are overcome by applying regular automatic defrosting (the high-temperature working fluid from the compressor, instead of flowing into the condenser, is recirculated into the evaporator) Modern air–water heat pumps can operate down to an outside temperature of − 15 °C However, then their COP is much lower than 3 and they no longer can meet the heating demand completely Air heat pumps operate in bivalent (dual) mode using an auxiliary heater in times of low outside temperature Usually, this bivalent mode is monoenergetic mode, that is, electric heater is used The heating water is preheated by the heat pump, to the selected flow temperature, and then an electric heating cartridge is used to provide auxiliary peak heat It should

be mentioned that an air–water heat exchanger must circulate a large volume of air (e.g., 3000–4000 m3 h−1) As a consequence, they can generate a lot of noise Air heat pumps can be constructed and installed as compact heat pumps and in this case air supply duct is used to supply outside air to the heat pump evaporator inside the building and exhaust air duct is used to take off the air used In this case, a problem with noise generation is very likely Therefore, the split construction is used very often, and the intake and outtake of air and evaporator are located outside the building, and other heat pump elements inside the building

There are also heat pumps utilizing exhausted air as a heat source They are applied mainly in buildings with very low energy demand, for example, in passive buildings, where they are coupled with domestic ventilation system with heat recuperation The

part of heat of air extracted from a building from the ventilation system that cannot be recovered in a direct way (in a recuperative heat exchanger) is used as a heat source for an integral exhaust air–water heat pump This type of a heat pump can also be used in other so-called ‘low-energy buildings’, but it cannot operate only in monovalent mode and an electrical supplementary heater is usually used to provide the additional auxiliary heating energy required to meet the total heat demand

The main advantages of air heat pumps, apart from very good availability of a heat source, are the following:

• heat extraction from outside air does not disturb the natural energy equilibrium of a heat source (environment) and its physical characteristics;

• high purity;

• there is no pollution because of extraction and exhaustion of air cooled, and no damage to the environment;

• there are very low costs of heat extraction, the lowest among all types of renewable energy source heat pumps, and the method of heat extraction is the simplest one

However, the main disadvantage, that is, no coherency between a heat source and a heat sink (space heating demand), causes the operation of a heat pump with low COP, and as a consequence a higher amount of electricity is used to drive the air heat pump than that required, for example, for the ground source one

The ground constitutes a suitable heat source for a heat pump considering small-scale low-temperature heating systems [7] The seasonal temperature fluctuations are much smaller than those of the ambient air even at small depths Ground at small depth is under the influence of solar radiation, rain, melt snow (water), and other environmental factors At a depth of 2 m, underground temperatures range from 2 to 13 °C during the heating season (in most European countries) With the increase of ground depth down to 10 m the temperature becomes nearly constant throughout the whole year and is approximately equal to the annual mean outside (ambient) air temperature [8] With the further increase of depth, the ground temperature increases but relatively very slowly The influence of geothermal energy is weak even at a depth of 50 or 100 m [9] The energy flowing from deeper layers upward represents only 0.063–0.1 W m−2

Energy stored in a natural way in the ground medium is extracted by means of large-area horizontal plastic pipework buried underground or longer length plastic tubes set into drilled vertical ducts or bore holes [7, 10] Heat exchangers are installed in an area next to the building In horizontal heat exchanger systems, the plastic pipes (e.g., polyurethane (PE)) are buried underground at

a depth of between 1.2 and 1.5 m Individual pipe runs (loops) are usually limited to a length of 100 m If the length of pipe runs is too big, then there is pressure drop in piping and as a consequence the required pump capacity would be too great All loops have to

be of the same length, because the pressure drop must be the same to achieve identical flow conditions in every pipe run As a consequence all heat exchanger loops can extract heat evenly from the ground medium Usually, heat extracted from the ground is transferred via water and antifreeze mixture (brine) to an evaporator of a heat pump Because of the liquid used, these heat pumps

are called brine/water heat pump (brine in the primary and water in the secondary (heating) circuit) In the ground outside the building or just directly inside the building, there is a header duct with a brine distribution that consists of two brine distributors, flow and return, where the pipe ends come together Return and flow headers are installed slightly higher than piping (venting) It is important that each loop is able to be shut off separately A circulation pump circulates the brine through the pipes that extract the heat stored underground The heat extraction from the ground varies from 10 W m−2 (in the case of underground areas of dry sandy soil) to 35 W m−2 (in the case of ground with groundwater ways)

In central European countries, the freezing zone in the soil is 1 m and in some regions 1.5 m deep More to the north (high-latitude countries) the freezing depth is bigger This makes it preferable to use vertical ground heat exchangers rather than horizontal ones [6] Heat is extracted from the ground by vertical ground heat exchangers that are mainly in the form of U tubes (so-called ground ducts or probes), double U tubes (so-called duplex probes), or concentric tubes (popular in the past, not at present) These tubes are coupled with a heat pump evaporator similar to the horizontal heat exchanger system Water and antifreeze mixture (brine) circulate in pipes and the pipe ends come together in a header duct with a brine distribution (with flow and return distributors) located outside or inside the building If a header duct is located outside the building, it is recommended to insulate the underground collector pipes (flow and return) The location of heat exchanger tubes can vary, and they can be set into the ground in rectangular, hexagonal, or cylindrical configuration The distance between vertical tubes depends

on the thermal and hydrological characteristics of the ground Usually, this distance can be at least about 4 m for single U tubes and

5 m for double U tubes Possible heat flux extraction for vertical ground probes depends on the type of the ground [7] and it can be

at a level of 20 W m−1 of a tube length for dry sediment with relatively low thermal conductivity (lower than 1.5 W m−1K−1) or even

70 W m−1 (of a tube length) for solid rock with high thermal conductivity (higher than 3 W m−1K−1) In the case of ground formed

by gravel and sand with waterways, the specific heat extraction for double U tube can be about 55–65 W m−1, and for moist clay and loam about 40 W m−1 Under standard hydrological conditions, an average possible heat flux extraction (so-called probe capacity)

of 50 W m−1 probe length can be expected (according to Reference [11])

The most important advantage of a ground heat pump, especially with vertical heat exchangers, is the fact that heating of a building can be accomplished in a monovalent mode of operation Of course, at the end of the heating season, due to heat extraction the ground medium is cooled down However, because of the natural heat and mass processes, that is, influence of ambient environment from the top, undisturbed ground surrounding the sides, and geothermal energy from the bottom, the ground can come back to its initial thermal balance This way, in an annual cycle, the natural thermal state of the ground source cannot be disturbed When bigger heat demand is expected, then it is very good to apply artificial charging of the soil, for example, by solar energy [12] (as described in Section 3.15.4), to ensure return to initial undisturbed thermal conditions Of course, when heating requirements of a building are rather small, then it is quite sufficient to apply ground heat pumps, using the ground as

a natural heat source without artificial charging of the ground medium

Ground source heat pumps can use the earth in direct or indirect mode In direct mode, heat exchangers buried horizontally or set vertically in the ground constitute the evaporator coils (evaporation process takes place just in the ground medium) When ground heat exchangers constitute separated closed loops or pipe runs and are coupled with a heat pump evaporator located inside a building, then a heat pump uses the ground in indirect mode It means that heat exchange between the ground body and working fluid in the heat pump evaporator (refrigerant) is accomplished through an additional medium In this case, a heat carrier fluid (brine), as an additional medium, circulates in the ground heat exchangers

The possibility of utilizing ground as a heat source varies from place to place and depends mainly on local geology and size of system To design a system it is good to make a general review of a place proposed for location and positioning of the ground system and to estimate heat demand and its distribution in time Having made this preliminary study on ground source potential and heating needs, the selection of the type and configuration of the ground heat pump can be made In most ground heat pump systems for small-scale applications, only very rough analysis is needed This analysis becomes more complicated when the size of the system, that is, heat demand, is bigger It is very useful to know the geological and hydrogeological characteristics of the ground

As a result, the thermal behavior of a ground system can be predicted more easily Geological and hydrological conditions are not so important in the case of small-scale ground-coupled heat pump systems When a high level of underground water (waterways) exists, then it is a great advantage for a ground system Heat from the surrounding undisturbed ground body at a higher temperature

is transferred more quickly to the ducts and probes of a ground system In the case of small systems, very often there is not much area available to install the system; therefore an analysis of disposition and arrangements of the system components, especially tubes of ground heat exchangers, is needed

As mentioned before, vertical ground heat exchanger systems are more recommended than horizontal coils, especially in high-latitude countries The main reasons are as follows:

• Heat extraction conditions are better deeper in the ground than just approximately 1 m below the earth surface, especially when freezing phenomena develop Deeper in the ground the temperature is higher (than just below the earth surface), and a heat pump uses the heat source of the higher temperature and operates in better thermal conditions As a consequence, the COP is higher (it can be at a level of 4–4.5), and hence a smaller amount of electricity is used to drive the heat pump

• Problem with ice formation on the evaporator surface (of a heat pump) does not occur; there is no need for defrosting The heating mode can be more predictable and stable

• The operation of a heat pump can be accomplished in a monovalent mode, as the heat pump is the only heating device providing all heating requirements

• If a part of heat exchanger tubes or one of them does not operate in an effective way or it is just out of order, this situation does not require to stop operation of the whole system (vertical tubes of heat exchangers are connected in parallel), as the other part of the system can overtake all heating requirements

• They do not need a large area of ground surface

• No excavation is needed (excavation can cause devastation of a big land (garden) area)

• They do not affect the plants growing and other gardening

• Maintenance of the system, including makeover, is more simple and convenient for the user

It should be added that nowadays drilling methods used to insert vertical tubes into the ground are well developed and prices are being continuously decreased Currently, the costs of borehole drilling including pipe inserting and working fluid infilling can lie between 30 and 50 € m−1 Depending on the technology used for given ground conditions and advancement of equipment applied, the drilling is usually done at a depth of 50–100 m, sometimes to 150 or even 200 m [13]

Nowadays, the majority of heat pumps that are installed are ground source heat pumps, mainly because the ground is available everywhere (however, the available ground surface can be different), and temperature and thermal capacity are predictable and they are relatively constant in time of a month or longer This makes ground source heat pumps very attractive for end users because they can operate in monovalent mode and their efficiency is relatively high Usually COP for ground source heat pumps with vertical heat exchangers is about 4–4.5 and for heat pumps with horizontal heat exchangers it is 3–4 To summarize, to design and select a ground source heat pump, there is a need to estimate the expected heating load and its nature, including demand level and its distribution in time, and expected temperature range, which can be different for space heating and DHW, and for any other use Apart from the specific heating requirements, a heat pump has to be selected according to its operation and thermal characteristics, ground thermal conditions, and expected efficiency of the ground source, including type and size of ground heat exchangers and their location and configuration

When analyzing geothermal water as a heat source, one should consider the deep geothermal energy resources (more than

1000 m deep) (are not dealt with in this chapter) and shallow geothermal energy sources (from a few meters to a few hundred meters) Ground water at considerable depth (aquifers) and shallow geothermal water are very interesting solutions for direct heating or for heating via a heat pump (depending on water temperature and heat flux that are carried by them) However, there are two main problems: high drilling and operating costs and very often poor water quality, for example, corrosive salt content Shallow geothermal water can be used with good efficiency if geohydrological conditions, including purity of water, are good enough to be used for heating purposes Most groundwater at depths of about 10 m is available at constant and relatively high temperature This constant temperature of between 7 and 12 °C (in central Europe) is maintained at a depth of several meters below the earth surface Groundwater is extracted through a supply well via a suction pump and transported to the evaporator of the water/water heat pump Subsequently, the cooled down water is returned via a return well This must be done in such a way as to not disturb water flows, and especially depletion of groundwater layers should be avoided A special regulation should be imposed to keep the necessary distance between a supply and return well Usually it cannot be smaller than 5 m (this is the minimum distance for small-scale applications) The purity of water is also very important Therefore usually there is an intermediate circuit heat exchanger that separates the water circuit of the supply and return wells from the evaporator circuit This type of heat pump system is suggested for small- to medium-scale applications However, groundwater heat sources can also be used at a larger scale, for example, when aquifers, groundwater reservoirs, are used as a heat source or for storage

Surface water constitutes a heat source that cannot be used in every location and is mostly used in medium-scale applications The temperature of surface water is strongly dependent on ambient temperature fluctuations; therefore, its possible application for heating purposes is limited in some climatic conditions The average river temperature in the coldest months can be about 0 °C, and the average lake temperature, which depends mainly on the depth of the lake, is even lower [14] For these heat sources, detailed analysis of the given location must be carried out In winter, surface ice formation can occur very easily However, with the increase

of depth, the dependence on ambient air temperature decreases Therefore, some systems of this type have two intakes of water One

of them is located at the surface of water and this is for the use in spring, summer, and autumn The other intake is located deep in the water, much below the freezing depth and this is for the use in winter

Surface water as a heat source for a heat pump is not used very often nowadays This heat source has too many disadvantages to

be popular The availability is not very good, because it is limited to few locations Coherency between the source and the user is rather poor, because it is strictly connected with ambient air temperature When space heating demand is high, the surface water temperature is low, because the ambient air temperature is very low The thermal capacity is quite big but the heat extraction can disturb the natural state of the source, especially in winter when freezing of water can develop very quickly, and the heat cannot be extracted In addition, the costs of heat extraction are usually significant Surface water (heat source) heat pumps have COP at different levels in time, the averaged COP for the whole year’s operation can be about 3 to 3.5

Solar energy is available during the daytime, but at different levels depending on geographical latitude, time of the day, and season of the year ‘Solar energy supply’ and ‘solar energy heating demand’ are quite opposite in time When energy is needed for space heating purposes, the time and peak values of heat demand are quite opposite to the time and peak values of available solar radiation However, a heat pump is a device that makes the application of solar energy for heating quite effective, as is considered in detail in the following section

Nowadays, the most popular heat sources for residential and commercial heat pumps are ambient air and ground Solar energy

as a heat source is not used so often However, recently ground source heat pumps have become increasingly popular and solar

collectors have been used more often not only for DHW, but also for space heating (combi systems) or even cooling (combi plus systems) It means that heat pumps and solar collectors ‘come together’ quite often in modern heating and cooling systems to provide all thermal energy needs In this way, they constitute combined solar thermal and heat pump systems, termed SAHP systems SAHP systems are planned and installed with different levels of complexity, using different components, and are applied for different heating and even cooling needs

3.15.3 Solar-Assisted Heat Pump System

3.15.3.1 Classification, Configurations, and Functions

The intermittent character of solar radiation and the strong dependence of the irradiation level on time of the day and season of the year make it necessary to store the solar energy and necessitate combining solar heating systems with other heating device to fulfill all heating requirements One such device is a heat pump Combining solar thermal systems with heat pumps is especially popular

in modern low-energy buildings, because such heating systems can supply all the heating demand and no auxiliary conventional heating is needed A heat pump replaces the burner or other conventional device or system used for space heating In summer when solar irradiation is high, a solar thermal system can provide nearly all heating demand for DHW use Combining a heat pump with solar thermal into one heating system can contribute significantly to the reduction of fossil fuel usage and as a result in reduction in the running costs of a heating system The other very important advantage is that such combination allows in most cases that the system operates in a monovalent mode, that is, not requiring any auxiliary heating device and as a result reduces fossil energy consumption The reduction of fossil fuel consumption is not only because of substituting the fuel (from fossil to renewable) but also due to better operation conditions of a heat pump, which results in higher COP and in consequence in reduced electricity demand (to drive the heat pump)

Nowadays, there are a variety of concepts of solar thermal and heat pump system combinations but there are not enough proven results to state definitely which one is the best Classification of SAHP systems is usually made because of the configuration of the system; mainly it is connected with the role of solar collectors and a heat pump for heating and the mutual interaction between them Traditional classification of SAHP systems is according to the heat source of a heat pump and its connection to the solar thermal part of the system, mainly solar collectors [15, 16] Based on that, the following categories can be classified: parallel, series, and dual-source SAHP systems In a parallel system, a heat pump uses a heat source other than collected solar energy and solar energy is supplied directly to the heating system or is stored in a storage tank The solar collectors and the heat pump are generally independent and can operate in parallel mode In a series SAHP system, solar energy directly from solar collectors or storage is used

as a heat source for a heat pump The heat pump is dependent on the solar collector operation In a dual-source SAHP system, a heat pump can use two heat sources Collected and stored solar energy is one of the heat sources and the other renewable heat source is usually ground or air The heat pump is only partly dependent on solar collector operation

There are three main functions of heating systems under consideration They are as follows: water heating (DHW), space heating, and space cooling The SAHP system can provide all these three energy needs or two or only one of them Nowadays, the monofunction systems (only for hot water or only for space heating) are used very rarely Usually multifunction systems are used They supply heat for hot water preparation (DHW) and for space heating Some such systems can also be used for cooling The cooling mode is mainly achieved via a heat pump or directly (see Section 3.15.2.2)

In the last decades of the past century, solar thermal systems were used only for hot water preparation Therefore in that time, SAHP systems were offered and installed with ‘traditional’ solar thermal part only for water heating for DHW system and with a heat pump for space heating [15–19] The concept of such system represents the idea of parallel SAHP system [17], where the solar thermal part for DHW was independent of the heat pump that was used only for space heating and it was based on a renewable heat source other than solar heat source In those days, because of the technology available and applied, the heat pumps were used mainly for space heating due to the requirement to operate in relatively stable conditions (space heating demand is needed continuously in winter and it is relatively constant during a day and night or even over a longer time span) DHW use is characterized by high fluctuation of heating demand during a day Without appropriate storage and automatic control (daily automatic timetable of hot water use), there would be sudden changes (increase) in a short time in the temperature level and the quantity of heat required for heating needs With the increase of the temperature difference between the heat source and heat sink (DHW system), and the increase of heat needed, the COP of a heat pump decreases Nowadays, construction of heat pumps and storage has been improved, automatic control is well developed, and heat pumps are used for all heating functions with high efficiency At present, there are many different configurations of SAHP systems The main components and loops of these systems can operate alternatively in series or in parallel mode; some have also extra function called active regeneration of the heat source (mainly ground and groundwater)

Usually, a heat source is used in an indirect way to supply heat to the evaporator of a heat pump It means that the heat source and the evaporator are integrated through an intermediate heat exchanger loop This loop transfers the solar energy collected or another renewable energy extracted from the environment to the refrigerant loop of a heat pump Sometimes, more intermediate loops and elements are used, for example, when a working fluid circulates between solar collectors and a heat exchanger in a storage tank, and then there is another heat exchanger in the storage connected with a heat pump evaporator Of course, every heat exchanger has a certain effectiveness, that influences (reduces) the final share of solar energy used Sometimes, the source of heat for the heat pump is used in a direct way It means that a solar collector and a heat pump evaporator are integrated into one unit The

evaporator is also the solar collector where direct evaporation of refrigerant takes place The heat output of a heat pump condenser can also be used in a direct way when the condenser is located directly in the space to be heated or in an indirect way through an intermediate heat exchanger loop

Liquid (water or antifreeze mixture) and air solar collectors can be used in SAHP systems The heat pump can also be of different type, for example, water–water, water–air, air–air, air–water, or brine–water In the case of air collectors, a storage tank is not always used, because they can provide heat directly to a heat pump evaporator When liquid solar collectors are used, they are always coupled with a storage tank Storage is a very important component of the heating system and it integrates all the other components [20] Storage tanks can contain intermediate heat exchanger loops or loops with direct evaporation or condensation Improvement

of the thermal performance of storage and reduction in storage volume can be achieved through utilization of not only specific heat

of the storage medium but also the latent heat gained during a phase change process (melting phenomena) of the medium For this, the storage tank can be filled with phase change materials (PCMs) that melt at a given temperature [21–23]

In operation, SAHPs can have high, low, or even zero interaction between the heat pump and solar collectors A heat pump and solar collectors can be coupled very strongly and solar energy can be even used for active regeneration of a heat source of a heat pump, as is applied in the case of ground and groundwater heat sources However, it shall be mentioned that the systematic classification of SAHP systems is still an open issue The interest in SAHP systems is growing, as evidenced by many new investments

as well as many new research tasks (e.g., International Energy Agency (IEA) Task 44 ‘Solar and Heat Pump Systems’, and IEA Solar Heating and Cooling Program) [24]

3.15.3.2 Direct Solar-Assisted Heat Pump Systems

In a direct SAHP system, solar energy is used directly to heat the working fluid of a heat pump, that is, a refrigerant A solar collector and a heat pump evaporator constitute one integrated unit Two types of solar collectors are used: unglazed solar collectors (bare solar collector) and regular glazed solar collector with one cover In this system, the working fluid of the heat pump, a refrigerant, flows through an integrated solar collector-evaporator Due to solar radiation incident on the integrated unit and the direct effect of the ambient surroundings, the refrigerant undergoes a phase change from liquid to vapor, so the evaporation process takes place at the location of the heat source, usually located on the roof of the building The refrigerant is directly evaporated in an integrated solar collector-evaporator Such system is called a direct expansion solar-assisted heat pump system (DX SAHP) This system represents also a split configuration

The main advantage of a DX SAHP system is the elimination of the intermediate heat exchanger that is required for standard SAHP systems with a closed loop of collector working fluid (antifreeze mixture), which makes the construction of the system simpler This also improves the thermal performance of the system only if it operates under appropriate weather conditions [25–27] A very important issue for DX SAHP system operation is the selection of a suitable refrigerant and it seems to be one of the main problems for the widespread use of such systems [25] Another very crucial and critical issue is the sizing of the collector– evaporator panels The thermal capacity of solar collectors (also evaporators) collecting solar energy should be matched to the heat pumping capacity of the compressor A collector–evaporator temperature can fluctuate rapidly The flexible operation of the direct SAHP system can be maintained by using compressor capacity modulation (a variable speed compressor) The refrigerant, especially during phase change from liquid to vapor during the evaporation process, should operate in relatively stable conditions However,

in a direct SAHP system, the temperature level can change rapidly in a short time, that is, during a few hours of a day, and over a longer span of time There are also huge differences in the temperature of working fluid in winter and summer; for example, in summer a refrigerant should evaporate at a temperature of 30 °C and in winter at a temperature of −20 °C Of course, it is possible

to find places (countries) with not so huge differences in temperature throughout the year The idea of DX SAHP is presented in

The following processes presented and numbered in Figure 7 take place:

1′–2′ Nonisentropic compression Slightly superheated refrigerant vapor (working fluid) at low pressure flows from the solar collector–evaporator into the mechanical compressor and is compressed to the required level of pressure and temperature

2′–2 Isobaric heat rejection Superheated vapor of the working fluid at high pressure and temperature flows from the compressor to the heat exchanger-condenser The heat rejection takes place due to temperature gradient and represents the desuperheating of the vapor

2–3 Isobaric and isothermal heat rejection – condensation Desuperheated vapor at high pressure flows through the heat exchanger–condenser Vapor condenses giving up the heat to the sink, that is, water flowing to a storage tank (then from storage to space heating circuit)

3–4′ Isenthalpic expansion Saturated (or slightly subcooled) refrigerant liquid from the condenser flows into the thermostatic expansion valve (throttle) and is expanded to low pressure and temperature (nonisentropic expansion process)

4′–1′ Isobaric and isothermal evaporation at the solar collector–evaporator due to solar radiation incident on the solar collector– evaporator Saturated liquid/vapor mixture at low pressure and temperature from the thermostatic expansion valve flows through the evaporator Under solar radiation the working fluid evaporates and at low pressure flows to the compressor, and the cycle repeats

A collector–evaporator is under the influence of the ambient surroundings, that is, solar radiation and ambient temperature The energy gain causes an increase in the enthalpy of the refrigerant and it can evaporate The energy balance of a solar collector– evaporator under steady-state conditions can be written as follows:

m h_ ð 1 ′ − h4 ′ Þ ¼F′Ac½GsðταÞ −ULðTf −TaÞ ½10 The term on the left-hand side of eqn [10] represents the energy gained by the working fluid (refrigerant) during evaporation (m is the mass flow rate, h is the enthalpy of the working fluid according to the state points in Figure 7) and it is equal to the solar energy gained by the solar collector–evaporator, represented by the term on the right-hand side, that is, the useful energy Qu gained from solar collectors (F′ is the efficiency of solar collectors, Ac the surface area of solar collectors, Gs the solar irradiation, UL the averaged loss coefficient) It can be assumed that the working fluid temperature Tf in a collector remains constant at the saturation value at constant saturation pressure (frictional pressure drop is neglected) Equation [10] can also be used in a quasistationary model of solar collector–evaporator operation During a day it is possible to observe that one quasistationary state follows the other At every quasistationary state temperature of the ambient air Ta and the working fluid temperature Tf of evaporation in a collecter, and solar irradiance Gs are stable, but they are different than in the following state

With the development of new technologies there are some novel ideas to integrate the DX SAHP system with other innovative technologies One of them is to replace the solar thermal collector–evaporator by PV/T (photovoltaic/thermal) collector–evaporator Some experimental studies on photovoltaic SAHP systems have been performed recently [28] PV/T collector–evaporator converts some portion of solar energy into electricity and some into heat Heat is absorbed by the refrigerant circulating in a heat pump loop and the refrigerant evaporates directly in a PV/T collector–evaporator The system also consists of the standard air evaporator, which is used when there is no or not enough solar radiation available (both evaporators are connected in parallel) There are also two condensers, air cooled and water cooled (also connected in parallel) The water-cooled condenser is used for space heating and DHW and the air-cooled condenser is used only for space heating There is a variable-frequency compressor and an electronic expansion valve in the system With changes in the frequency of the compressor, the input power

to drive the compressor also changes To meet the changes in the operating frequency of the compressor, the expansion valve adjusts its position automatically The PV DX SAHP system generates heat and electricity; therefore, it is proposed to use a comprehensive coefficient of thermal and electrical performance COPp/t, which is defined in the following way [28]:

The idea of application of direct SAHP systems was presented many years ago [29], and from time to time some researchers call for the comeback of this technology [25–28] However, up to now, this technology has not become popular There are some advantages of direct SAHP systems, but perhaps disadvantages, with the main one being uncertainty of their operation in unstable weather conditions, are so strong that they limit significantly the interest in the implementation of these systems in practice 3.15.3.3 Series Solar-Assisted Heat Pump Systems

A series SAHP system could be just called a solar heat pump, because solar energy is the only heat source used for the heat pump In most series SAHP systems, there are two main modes of system operation (it could also have auxiliary heating, and then it can be considered as a third mode of operation):

• solar direct heating – if the temperature of heat collected or stored (depending on the system configuration) is high enough, then solar energy is used directly for heating;

• solar indirect heating via a heat pump – if the temperature of heat collected or stored is too low for direct heating, then a heat pump is used to meet the heating demand, that is, solar energy converted into useful heat is used as a heat source for a heat pump

It should be mentioned that in the middle- and high-latitude countries, mainly liquid solar collectors are used (flat plate or vacuum tube) with the antifreeze mixture circulating in a collector loop and a storage tank, with water as a storage medium A storage tank is

a central component of the heating system Storage of heat improves the thermal performance of a system and increases its reliability [20, 30] The location of a storage tank can differ depending on the system configuration, which influences the mode of operation To improve the ability to store heat, PCMs can be used (the storage tank is filled with PCM) and then heat storage takes place due to both the specific and latent heat of the water and PCM storage medium [21–23] In low- and middle-latitude countries, liquid, mainly water, or air solar collectors are used

In the case of air collectors, they are usually applied for space heating of a building, mainly by passive operation, and they are usually integrated with the south façade of a building The outdoor (ambient) air circulates in spaces or channels within the solar collectors’ south facade The air collectors can also be used actively when fans are used for forced circulation of the air or when they (collectors) assist the evaporator of an air–air or air–water heat pump If the solar air collectors of a solar passive system are used to supply space heating directly very often in winter even in a warm climate (like the Mediterranean), they do not operate all the time, because of too low ambient temperature and too low solar irradiation level [31] However, if these collectors are used to supply outside air heated by solar radiation as a heat source for a heat pump, then they can be used all year long and there is no need to use any other heating device This type of series SAHP systems can operate in a hybrid mode, that is, they are based on passive (collectors)–active (heat pump) operation at the same time If a heat pump is used only for space heating, it can be an air–air heat pump However, if it is used for a few functions, mainly space heating and cooling and DHW, then the air–water heat pump can be used, and space heating and cooling can be accomplished, for example, by fan coils

The idea of series SAHP systems with solar collectors was introduced at the end of the 1970s [18] An example of a traditional series SAHP is presented in Figure 8 The numbers given in the figure are for the main components of the system In the series SAHP system, the heat carrier working fluid (water or antifreeze mixture) circulates in a solar collector loop and transfers heat collected by solar collectors to a heat exchanger in the storage tank There is another heat exchanger in the storage tank that couples this storage tank with a DHW storage tank and/or with the heating circuit of a building or with a heat pump evaporator

It means that the DHW system is theoretically independent of the space heating and the DHW storage tank is equipped with an auxiliary heater Heating of the building can be accomplished directly from the storage tank or via a heat pump The heat carrier fluid, which is usually water, transfers heat from the storage tank to the heat pump evaporator Heat pump refrigerant takes heat out of the water, which after being cooled down comes back to the storage tank Heat from a heat pump condenser is extracted by the working fluid of a heating circuit to provide heat to the building Depending on the type of heating system, water or air is used

as the space heating fluid

To describe the operation of the standard SAHP system presented in Figure 8, the energy balance of the storage in unsteady state can be represented as

9

20 Figure 8 Standard series SAHP system Legend: 1, solar collector; 2, heat exchanger; 3, storage tank; 4, heat pump 5, space heating circuit; 6, ground heat exchanger; 7, three-way control valve; 8, temperature sensor; 9, circulating pump 10, DHW storage tank; 11, safety valve; 12, expansion tank; 13, solar control; 14, temperature sensor in a collector loop 15, bleed; 16, nonreturn valve; 17, temperature sensor; 18, temperature sensor in a ground loop 19, circulating pump in a ground loop 20, control; 21, cold water supply; 22, main storage tank; 23, auxiliary heater

 

dTs

ðVcρÞdt ¼ QuðtÞ − QlossðtÞ − QhdðtÞ − QhpðtÞ − QDHWdðtÞ ½12 The term on the left-hand side of eqn [12] expresses the storage capacity of a storage medium in a tank (Vcρ (J K−1)) and the fluctuation of storage temperature Ts in time caused by (terms on the right-hand side) useful solar energy gains Qu supplied by solar collectors, which is reduced because of the heat losses from the storage Qloss (with high-quality thermal insulation of the tank, the losses can be neglected) and the heat used directly for space heating purposes Qhd and for DHW heating demand QDHWd (heat supplied to a DHW tank) and because of heat supplied to a heat pump evaporator Qhp The storage temperature Ts in eqn [12] refers

to storage with full mixing of storage medium (water) or can be treated as the averaged value if there is stratification effect in the storage tank

The main modes of operation of the series SAHP system under consideration (the heat losses from the storage are neglected,

Qloss = 0) are as follows:

• Solar DHW heating: Heat stored in the storage tank is transferred to the DHW tank When temperature TsDHW of stored heat in the DHW tank is high enough, that is, if TsDHW > TDHWmin, then all DHW load is supplied from the DHW tank, QDHWd = QDHW, and the auxiliary heater is turned off, so Qaux = 0 When the temperature of water in the DHW tank is too low, TsDHW < TDHWlimit, then the auxiliary heater is on and supplies the rest (auxiliary) of heat, thus QDHW = QDHWd + Qaux When the temperature of water in the DHW tank is below the minimum level, TsDHW < TDHWmin, then the auxiliary heater is on and supplies all DHW demand,

QDHW = Qaux Depending on the solar radiation level and the difference between the temperature of solar collectors and the main storage, the solar collector loop can operate or not

• Solar direct heating of the building, when solar energy is used directly for heating the building: When temperature Ts of stored heat is high enough, that is, if Ts> Tsmin, then Qhd = Qheat and the heat pump is turned off, so Qhp = 0; depending on solar irradiation and the difference between the temperature of solar collectors and storage, the solar collector loop can operate (Qu > 0)

or not (Qu = 0)

• Solar indirect heating of the building via the heat pump: When the temperature of collected or stored heat is too low for direct heating, that is, if Ts ≤ Tsmin, then the heat pump is used and supplies heat to meet space heating demand Taking into account eqn [2c], the heat extracted out of the storage tank as a heat source of the heat pump is equal to

1

COPhpðtÞ and heat supplied to a building is extracted from the heat pump condenser, so Qhpcon = Qheat and Qhd = 0, and Qu > 0 or Qu = 0, depending on whether solar collectors operate or not

• Solar indirect heating of a building via a heat pump and auxiliary heating: When the heat pump is used to supply heat but it cannot provide all heating requirements (e.g., there is a limit for the lowest COP value), then the auxiliary heater is used to supply the rest of the space heating demand, and Qhpcon + Qaux = Qheat

The COP of a heat pump is given in general form by eqn [2c] However, the COP is also applied to determine the thermal performance of the whole SAHP system The COP of the whole SAHP system is defined as the ratio of the total heat received to the total work input to the heating system Thus, in the case of a series SAHP system used only for heating a building, the COP can be expressed in the following way:

Qheat Qhd þ Qhpcon mcCp½ðTsout − Tsin Þ þðTconout − TconinÞ

Wtotal Wtotal Wþ WpumpSd þ WpumpShp þ Wheat

The numerator of eqn [14] represents the quantity of heating energy at the heat sink, that is, Qheat, that is supplied by the solar system directly Qhd (Tsout is the supply temperature at the outlet of the storage and Tsin is the return temperature at the inlet of the storage tank) and by a heat pump from the heat pump condenser Qhpcon to the heating circuit (water or air of specific heat Cp and mass flow rate mc; Tconout and Tconin are the temperature at the outlet and inlet of the condenser, respectively) The denominator represents the total work input into the SAHP system, that is, not only the work input W needed to drive the compressor of a heat pump, but also the work input WpumpSd for the circulating pumps of the solar heating system (without DHW part) during direct heating mode, the work input WpumpShp for the circulating pumps of the solar heating system (without DHW part) during heating via a heat pump, and the work input Wheat for the circulating pumps of water heating circuit in a building or for fans in the case of the air heating system

The COP of the series SAHP system under consideration can be also written taking into account the auxiliary heating; then, the numerator of eqn [14] represents the quantity of all heating energy supplied to the building Qheattotal, that is, by the solar system directly Qhd, by the heat pump from the heat pump condenser Qhpcon, and by the auxiliary heater Qaux The denominator represents the total work input including the work input Waux for auxiliary electric heater and eqn [14] takes the following form:

Qheattotal Qhd þ Qhpcon þ Qaux

Wtotal Wþ WpumpSd þ WpumpShp þ Wheat þ Waux

The COP given by eqns [14] and [15] should be applied only for the space heating system of the SAHP system considered This is due to the fact that a heat pump is coupled directly with a solar thermal system only for space heating needs The DHW heating system constitutes in some way an independent solar thermal system with its own auxiliary heater The DHW system operates in parallel to the SAHP space heating system Therefore, even if there is a common storage tank for both systems (in result there is some interaction between the two systems), in the case under consideration it is better not to include operation of the DHW solar heating system in the determination of the COP of the series SAHP system

In one of the very simple forms of a series SAHP system, the solar energy collected by the solar collectors is used only as a heat source for the heat pump and there is no other mode of operation, so there is no direct supply of solar energy (collected by solar collectors) to the heating system [32] In such system, there are two storage tanks: a low-temperature storage tank and a high-temperature storage tank The low-temperature storage tank is located between a solar collector loop and a heat pump evaporator and the high-temperature storage tank between a heat pump condenser and a space heating circuit in a building This idea was also checked experimentally [33] A heat pump evaporator can be situated directly in a low-temperature storage tank and a heat pump condenser directly in a high-temperature storage tank However, it is also possible to use heat exchangers in low- and high-temperature storage tanks to be coupled with a heat pump evaporator and condenser, respectively Of course, an auxiliary peak heater can be also included in the system, for example, in a high-temperature storage tank

There are different modifications of the series SAHP system One of them is realized through introduction of long-term storage separate from short-term storage However, such system is no longer a typical series SAHP system It should be mentioned that in the past hot water preparation (DHW) used to be an independent function (from space heating) and DHW was provided by solar energy and auxiliary heater (usually electrical) in parallel to space heating, which was accomplished by the series SAHP system Heating of a building and heating of DHW were then independent Nowadays, hot water preparation is one of the standard functions of the SAHP system Usually, storage (with stratification) is a central component of the heating system and space heating and DHW are supplied by a combined heating system The automatic control system manages the heat supply to DHW and space heating or eventually to cooling or air conditioning system

3.15.3.4 Parallel Solar-Assisted Heat Pump

A parallel SAHP system consists of a solar thermal part and a heat pump that uses a heat source other than solar energy [17] Solar liquid (water or antifreeze mixture) collectors or solar air collectors can be used In a solar heating system based on liquid solar collectors, solar energy can be used directly for heating purposes or through a storage tank, and an auxiliary heater can also be applied A heat pump uses usually the ambient air or ground as an independent heat source If solar air collectors are used, they are applied mainly for passive heating of a building, but because they cannot usually fulfill the space heating requirements in cold days (even in a warm climate) the active heating of the building is realized through air-to-air or air-to-water heat pump [31]

As it has been mentioned, in the past a solar heating system (solar collectors and storage) was responsible only for hot water heating (DHW) and a heat pump for space heating Both systems used to operate without interaction A standard parallel SAHP system is presented in Figure 9 The legend to this figure is the same as for Figure 8

This system consists of conventional solar thermal part with solar liquid collectors (water or antifreeze mixture) in a closed solar collector loop and a storage tank (If air collectors are used, they are integrated into the building façade and the solar collector loop is open.) There are also heat exchangers in a storage tank for DHW and for space heating There is another heat exchanger in the storage tank that couples this storage with a DHW storage tank, in a similar way as a series SAHP system presented in Figure 9 The DHW