Volume 3 solar thermal systems components and applications 3 12 – solar hot water heating systems

Volume 3 solar thermal systems components and applications 3 12 – solar hot water heating systems Volume 3 solar thermal systems components and applications 3 12 – solar hot water heating systems Volume 3 solar thermal systems components and applications 3 12 – solar hot water heating systems Volume 3 solar thermal systems components and applications 3 12 – solar hot water heating systems Volume 3 solar thermal systems components and applications 3 12 – solar hot water heating systems

G Faninger, University of Klagenfurt, Klagenfurt, Austria; Vienna University of Technology, Vienna, Austria

© 2012 Elsevier Ltd All rights reserved

References

3.12.1 Toward a Sustainable Energy System

3.12.1.1 Solar Heat – Renewable Energy Source with High Potential

The facts of our present energy supply – limited fossil resources, instability by political influence on the oil and gas markets, and greenhouse gas emission from fossil energy resources – are serious arguments for creating a new energy system The main resources for a future sustainable energy system will be renewable sources And, solar thermal technologies have the potential for a high contribution to the future energy supply

The ‘solar source’ for solar thermal systems is immense and inexhaustible The environmental and economic benefits are substantial

Today, solar thermal systems are regarded as a well-established, low-tech-technology with an enormous potential for energy production ‘Solar thermal technologies’ for low- to medium-temperature applications can be used all over the world – cold to hot climates A large variety of solar thermal components and systems, mostly for residential applications, are available in the market The products are reliable and have a high technical standard in the low-temperature regime (below 150 °C)

There has been a rapid market growth in recent years for small solar hot water systems in countries moving toward partly automatic or semiautomatic fabrication of solar thermal components

Solar thermal systems in larger buildings – multifamily houses and apartment blocks – as well as in district heating plants are now emerging in the market The use of solar hot water systems in larger buildings and centralized solar thermal systems has the

Market deployment Swimming pool heating

Domestic hot water

Hot water in multifamily housing

District heating Solar Combisystems Facade collector systems

Sea water desalination Process heat

Cooling

Research and development Market introduction Market deployment

advantage of lower specific investment costs, and thus, the heat production costs can be reduced in comparison with small, decentralized systems The possibilities for a central hot water preparation in multifamily buildings are used increasingly in the market nowadays

The component for the conversion of solar energy into heat is the collector – either nonconcentrating or concentrating Collector working temperatures of about 60–80 °C, with conversion efficiency from 40% to 60%, can be achieved with flat-plate collectors, which are typically used for hot water solar systems The properties of this type of collectors are well known today and thus manufactured in many parts of the world In countries with solar radiation ≥1800 kWh m−2 yr−1, it is advantageous to use solar systems for domestic hot water (DHW) preparation as compact system with flat-plate collectors based on the thermosi­phon principle Synthetic absorbers are preferred to metal absorbers not only for cost reasons but also due to lower corrosion potential

Solar heating and cooling (SHC) technologies include solar water heating, solar space heating and cooling, using active technologies and passive system designs, daylighting, and agricultural and industrial process heating The use of solar energy in housing presents remarkable advantages as follows: requires less energy; causes less adverse environmental impacts, for example, CO2; provides open sunlight; improves building esthetics; and provides a new medium for archi­tectural expression

While solar water heating and solar space heating have been in the market for decades, new approaches for solar thermal applications (e.g., for cooling and process heating) are now emerging in the market Solar-assisted cooling is an extremely promising technology as peak cooling requirement coincides with peak solar radiation Small-scale solar cooling systems are now commer­cially available

Figure 1 illustrates the market development from solar thermal technologies

3.12.1.2 Solar Water Heating

Today, DHW preparation with solar energy is standard in many countries In the area of building renovation, solar hot water preparation is attractive to increase the efficiency of heating systems Especially, ineffective heating systems for hot water preparation outside the heating season have been replaced by solar hot water preparation Thus, pollutant emissions through heating (wood, coal, and oil boilers) could be reduced, and at the same time, a high comfort in hot water preparation could be reached

Solar hot water preparation in high-performance houses is sensible In such houses, the energy needed to heat domestic water can equal or even exceed the energy needed for space heating, since the latter has been so far reduced by insulation and heat recovery In Europe, about 50% of the new detached and row houses and about 15% of apartment houses are designed on this concept

Figure 1 Solar thermal technologies in the market: From research to demonstration and market deployment

Further, demand for heating domestic water is a 12-month energy demand, including the high insulation during the summer months Using a solar system is therefore an effective way to reduce the total primary energy demand Increasingly, the market for solar water heating systems also includes systems that provide, in addition to hot water preparation, space heating in winter, called

‘Solar Combisystems’

For hot water heating in transition countries, such as China and India, and also in countries without space heating systems (e.g., Greece, Cyprus, and Malta), direct electricity is used Large amount of electricity is necessary to meet the hot water requirements in domestic, institutional, and commercial sectors resulting in peak load and load shedding to the shortage of power supply With solar hot water systems, the electricity demand as well as the peak load can be reduced remarkably (Figure 2)

Figure 2 Solar hot water systems to replace electricity demand and to reduce peak load

Figure 3 Solar heat for developing countries

3.12.1.3 Solar Energy for Developing Countries

The utilization of solar energy is considered to be promising in developing countries with suitable meteorological conditions Also, the potential for decentralized (stand-alone) energy systems is huge in developing countries Therefore, the use of solar energy for heat and electricity production is the first step for economic development (Figure 3)

It appears essential to promote the development, testing, demonstration, and market introduction of solar technologies in developing countries with the support of industrialized countries Many joint projects were initiated since 1980, with the govern­mental support of OECD-Member States, the World Bank, UNIDO, and other organizations

3.12.1.4 Market Introduction and Market Deployment of Solar Thermal Systems

As a result of the first oil price crisis, the market introduction of solar hot water systems started in most of the industrialized countries in 1976 with the aim of consumers to reduce the dependency from oil imports (First Solar Boom) From 1980 until the mid-1990s, the solar market development was not stable Initially, the collectors and systems were offered by small companies, but due to missing guidance information for design and construction, the consumers were not always satisfied The market deployment decreased, but through new firms and better-educated installers and available experiences on the market, the amount of installed collectors and systems increased again in late 1970s (Second Solar Boom) The situation on the solar thermal market for Austria is illustrated in Figure 4 Favorable applications were the separation of hot water preparation in households from firewood heating systems in small communities, especially outside the heating season With the decrease of oil price at the beginning of 1980, the solar market decreased again In this period, ‘self-built’ solar heating systems were organized, primarily for solar projects for personal use, and were offered in the market Through these private activities, the interest for solar systems was pushed and industry was motivated for more attention and new activities (Figure 5) From early 1990s onward, larger solar firms were found, and the industrial production was based on national standards, guidance for energy-efficient design, construction, and operation With the increase of industrial produced

Figure 4 Market deployment of solar thermal collectors in Austria

collectors, the production of self-built collectors and systems was focused to ‘social’ projects – to involve unemployed young people as well as handicapped persons with the aim to open perspectives for the job market The products are used in social projects

More attention for ‘greenhouse gases’ and their potential for climate change were given – both in policy and by consumers – and this supported the solar market remarkably at the end of 1990s (Third Solar Boom) Today, solar hot water systems are well designed, using materials with an expected lifetime of more than 25 years; the price for installed systems is acceptable; and the results are satisfying the consumers Also, financial support by the governments has influenced the increase of annual growth rates

3.12.1.5 Solar Heat Worldwide

Installed solar thermal capacity grew by 9% around the world in 2007 Solar thermal power output reached 88 845 GWh, resulting in the avoidance of 39.3 million tons of CO2 emissions At the end of 2007, the installed solar thermal capacity worldwide equaled 146.8 GWth or 209.7 million square meters The breakdown by collector type is as follows: 120.5 GWth for flat-plate and evacuated-tube collectors, 25.1 GWth for unglazed plastic collectors, and 1.2 GWth for air collectors (Figure 6) [1, 2]

3.12.1.5.1 Distribution by application

The use of solar thermal energy varies greatly by country In China and Taiwan (80.8 GWth), Europe (15.9 GWth), and Japan (4.9 GWth), plants with flat-plate and evacuated-tube collectors are mainly used to prepare hot water and to provide space heating, while in North America (the United States and Canada), swimming pool heating is still the dominant application with an installed capacity of 19.8 GWth of unglazed plastic collectors It should be noted that there is a growing unglazed solar air heating market in Canada and the United States aside from pool heating Unglazed collectors are also used for commercial and industrial building ventilation, air heating, and agricultural applications Europe has the most sophisticated market for different solar thermal applications It includes systems for hot water preparation, plants for space heating of single-family and multifamily houses and hotels, large-scale plants for district heating, as well as a growing number of systems for air conditioning, cooling, and industrial applications

From the worldwide collectors capacity in operation (2007), 50% are evacuated-tube collectors, 32% flat-plate collectors, 17% unglazed collectors, and 1% air collectors (mainly from the ‘SolarWall’ type) The main markets for evacuated-tube collectors are in China, while most flat-plate collectors are found in Europe In the United States and Australia, unglazed collectors are dominating But in recent years, the worldwide market for new installed glazed collectors has been significantly growing, in Europe with growth rates near and above 100% compared to the capacity installed in 2006

Self-built collector

Second Solar-Boom

Driven by Gases"

"Greenhouse-Third Solar-Boom

Supported by market-proofed technologies and with financial governmental support

050,000

Figure 5 Development of collector production and installation

The already installed capacity of solar thermal heat is considerably higher than the installed capacity of the other renewable sources The total energy yield of solar thermal heating systems comes in second place behind solid biomass, but it is higher than the energy yield of wind and photovoltaic (PV) power

Europe: EU-27, Albania, Macedonia, Norway, Overseas Departments of France, Switzerland

Others: Barbados, Brazil, India, Israel, Jordan, Mexico, Namibia, South Africa, Tunisia, Thailand, Turkey

Total capacity in operation of water collectors

of the 10 leading countries at the end of 2007

Turkey Germany Japan Australia Israel Brazil Austria Greece

Glazed Unglazed

Total capacity of glazed flat-plate and evacuated-tube collectors

in operation by economic region at the end of 2007

59.8

50.4

38 31.5

9.5 5.3

China and

Taiwan

Australia and

New Zealand

Japan Europe Others United States

and Canada Collector yield per 1000 inhabitants (kWh

18.67

8.609

4.31

China and Taiwan Australia and New Zealand

Japan Europe Others United States

and Canada

Worldwide capacity in operation 2007

by collector type

Unglazed collector 17%

Air collector 1%

Flat-plate collector 32%

power Photovoltaic Solar thermal

power Ocean tidal power

Total capacity in operation (gwel) Produced energy (twh)

Figure 6 Worldwide solar thermal market 2007 Source: Solar Heat Worldwide, 2009 Edition

To find a more detailed analysis on the market penetration of solar thermal technology in the 49 documented countries representing more than 85% of the solar thermal market, see http://www.iea-shc.org [1]

3.12.2 Technologies for Solar Hot Water Systems

The key applications for solar thermal technologies are those that require low-temperature heat, such as for swimming pools, for DHW and space heating, drying processes, and process heating in the low- to medium-temperature range

Solar water heating, including pool heating, has been commercially available for over 30 years, and can be considered a mature technology Active solar space heating, while commercially available for almost as long, significantly lags behind

solar water heating in the market due to its relatively higher costs as well as special requirements for utilization (only low-energy buildings with low-temperature heat distribution) But in recent years, systems that combine water and space heating, called Solar Combisystems, have emerged in the market and show great promise for further market success [3, 4]

Solar heating systems for combined DHW preparation and space heating are similar to solar water heaters in that they use the same collectors and transport the heat produced to a storage device There is, however, one major difference; the installed collector area is generally larger for Solar Combisystems, and in addition, this system has at least two energy sources to supply heat: the solar collectors and the auxiliary energy source The auxiliary energy sources can be biomass, gas, oil, or electricity This dual system makes Solar Combisystems more complex than solar DHW systems with the additional interactions of the extra subsystems These interactions profoundly affect the overall performance of the solar part of the system

Figure 7 illustrates examples of solar heating systems

3.12.2.1 Components and Concepts

The components of a solar DHW system are collector, storage, collector cycle, heat exchanger, auxiliary heat source, and regulation

Solar systems for DHW system are fairly simple and manufactured and marketed today in developed as well as in developing countries

Two different principles for solar DHW systems are used:

1 Systems with natural circulation

2 Systems with forced circulation

Figure 8 shows the principal schemes of solar water heating systems

3.12.2.1.1 Solar DHW systems with natural circulation

Solar DHW systems with natural circulation (thermosiphon systems) are most favorable in areas with a mean annual sum of global radiation on a horizontal surface above 1800 kWh m−2 yr−1 Thermosiphon systems can work satisfactorily only if the storage tank is mounted above the collector and if the collector warms up enough to establish a density difference between the water in the collector and the water in the storage tank The density difference is a function of the temperature difference, and therefore, the flow rate is a function of the useful gain of the collector that produces the temperature difference The systems are self-adjusting with increasing gain leading to increasing collector flow rates (Figure 8)

The efficiency of heating systems during summer months could be improved by larger storage volumes or by hot water extraction during the day If a constant water temperature is needed at any time, a backup heating system must be incorporated in the system Due to the meteorological condition in most of developing countries – solar radiation ≥1800 kWh m−2 yr−1 – solar hot water systems according to the thermosiphon principle are suitable for domestic use and can be manufactured at a reasonable price Because of the high lime and salt content of the tap water, special attention has to be paid to possible calcification and corrosion The rubber absorber made of polymeric materials (e.g., ethylene propylene diene monomer (EPDM)) turned out to be useful It is recommended to use glass material for covering purpose, because plastic covers tend to decolorize, which results in a reduction of the solar radiation absorbed

Solar hot water systems with collector areas exceeding 10 m2 should be supplied with forced circulation It should be possible to mount the collectors on flat roofs without expensive auxiliary structures, which reduces investment costs and improves economic application considerably

3.12.2.1.2 Solar DHW systems with forced circulation

Solar DHW systems with forced circulation are the common concepts in areas with moderate and cold climates The components of

a compact solar DHW system with forced circulation – for a household/single-family house – are shown in Figure 9

3.12.2.2 Solar Thermal Collectors

Collectors are the component for the conversion of solar energy into low- and high-temperature heat ‘Nonconcentrating’ collectors fully utilize the global radiation but ‘concentrating collectors’ use only the direct beam component of the radiation by concentrating irradiation on the absorber, thus increasing the intensity of radiation on the absorber Concentrating collector systems are the preferred technology in regions with more than 2500 annual sunshine hours (Figure 10)

The simplest design of a nonconcentrating collector is the ‘flat-plate collector’ The properties of this collector are well known As absorbers, black painted metal (copper, aluminum, or steel) or plastic plates are used and in order to reduce the useful heat losses – which increase with rising temperatures – transparent covers are placed on the collectors and appropriate insulation is provided at the back side of the absorber (Figure 11) With this type of collector, temperatures up to 80 °C with conversion efficiency of about

40–60% can be achieved Applications of this type of collector are swimming pool heating, water heaters, agricultural drying, desalination, and space heating

Solar hot water Solar Combisystem

Figure 7 Examples for solar thermal systems for low- to medium-heat production

Collector

Solar compact system for hot water preparation

Hot water (1),(2) Collector-pipes

Cold water Collector

Auxiliary Thermosyphon system

Hot water

Storage tank Collector pipe

Collector

Cold Water

Schematic of natural circulation

solar water heater

Figure 8 Solar domestic hot water (DHW) systems with natural circulation

Figure 9 Components of a compact solar domestic hot water (DHW) system for a household/single-family house

For temperatures above 100 °C, advanced designs, like some ‘evacuated-tube collectors’, have been developed To obtain fluid temperatures above 150 °C, ‘concentrating solar collector’ systems must be used The concentrator (a mirror or lens) is normally equipped with a tracking device that follows the sun The absorber in this system is located close to the geometric focus of the concentrator to intercept most of the incident direct radiation In general, there are two types of concentrators: (1) the linear focusing concentrator and (2) the point focusing concentrator In summary, the type of collector to be used

(a)

Collector types and working temperatures

=> Concentrating collector Collector type

Advanced flat-plate collector, evacuated-tube collector Flat-plate collector, CPC collector

Plastic absorber

Working temperatures

0 °C 50 °C 100 °C 150 °C 200 °C 250 °C

(b)

Collector types for solar thermal systems

Swimming pool (outdoor)

Hot water and space heating Transparent covered flat-plate collectors, Temperature = 80 °C CPC collectors, and evacuated-tube collectors

Air-conditioning and cooling Advanced flat-plate collectors

Process heat Temperature � 200 °C

Advanced flat-plate collectors, evacuated-tube collectors, and concentrating collectors Figure 10 Collector types for low- to medium-temperature applications (a) Collector types and working temperatures and (b) collector types for solar systems

depends on the application and the desired temperature For DHW preparation, flat-plate collectors with selective coating are the most cost-effective solution For higher temperatures (above 80 °C) and lower solar radiation, evacuated-tube collectors would be the better choice

3.12.2.2.1 High-performance flat-plate collectors

A high-performance flat-plate collector is characterized by a superior absorber and glazing The absorber should have a coating with

a high solar absorptive black painting (>95%) and low heat emissive selective coating (<5%) The glazing should be antireflection treated and consist of a low iron glass type to maximize solar radiation transmitted to the absorber Such flat-plate collectors can easily achieve outlet temperatures of 80 °C with a conversion efficiency of about 50–60%

Evacuated-tube collectors achieve superior performance because the vacuum surrounding the absorber drastically cuts heat losses to the ambient Outlet temperatures above 100 °C are easily achieved with a higher conversion efficiency compared with a flat-plate collector The inside-facing underside of the glass pipe has a reflective coating to irradiate the absorber from beneath Thus,

Figure 11 Design of a flat-plate collector

vacuum collectors have the further advantage of not having any given slope for optimal performance The glass pipes can simply be rotated to the optimal incident angle for the application For this reason, they can be mounted on a south facade or roof 3.12.2.2.2 Properties of collectors

Collector can be characterized by means of two experimentally determined constants:

1 Conversion factor: The collector efficiency when the ambient air temperature equals the collector temperature

2 Heat loss coefficient: The mean heat loss of the collector per aperture area for a measured temperature difference between the collector and the ambient air temperature in W m−2K−1

These collector constants are determined under exactly defined conditions (global radiation intensity, angle of incidence, air temperature, wind velocity, etc.) The performance of different collector types and applications are shown in Figure 12 The efficiencies are given for the values: temperature difference between the collector and ambient divided by the solar radiation Logically, as the collector gets hotter, the efficiency falls off For heating of high-performance houses, selective coated collectors or vacuum pipe collectors are a good choice

3.12.2.2.3 Integration of solar collectors

It is beneficial to integrate solar collectors into the building envelope for esthetic and economical reasons, and when doing so,

it is essential to take into consideration the architectural rules and local building traditions Building-integrated collectors are illustrated in Figure 13 Facade collectors are used in urban buildings, where sufficient suitable and oriented roof for the installation of solar collectors is not available A collector element directly integrated in the facade presents both solar collection and heat insulation of the building envelope The advantages of facade-integrated collectors are cost savings as a result of joint use of building components, replacement of the conventional facade, and the collectors suitable for both new and existing buildings

(1) Collector frame(2) Insulation(3) and (4) Transparent cover(5) Absorber(6) Pipes for heat transfer medium(7) and (8) Inlet/outlet of heat medium

DirectSolar radiation

AbsorberTransparent cover

7

Figure 12 Efficiencies of different collector types under different conditions and appropriate uses Tk, Collector working temperature (°C); Tu, ambient temperature (°C); G, solar irradiation (W m−2)

For roof installations, where the systems deliver heat over the whole year, the optimal tilt angle (northern hemisphere) is between 30° and 75° The orientation can be between 30° east and 45° west Facade-integrated collector is far from optimum in all locations (see Section 3.12.3.3) However, it performs better in winter by low sun angles When there is snow cover, it receives an extra portion of ground-reflected solar radiation Roof collectors with too little slope, by contrast, will have zero output when covered by snow A big limitation of facade collectors is, however, that by low sun angles, neighboring buildings and trees will cast shadows on the collector surface

3.12.2.2.4 New developments in the collector sector

The objective of new developments in the collector sector is the cost reduction as well as durability and reliability of novel design of solar thermal systems Polymer engineering and science offers great potential for new products and applications, which simultaneously fulfill the technological and environmental objectives as well as social needs The full potential of polymeric materials can only be used when several product functions are integrated into a single component in a fundamentally new design These goals will be achieved by either less expensive materials or less expensive manufacturing processes

The most common nowadays is the use of copper absorbers for flat-plate solar collectors The copper content in conventional flat-plate collectors varies between 2 and 6 kg m−2 Taking into account the copper used in piping and heat exchangers/heat stores,

5 kg m−2 collector may be a good estimate Each square meter of collector delivers about 300 kWh heat per year Hence, 1 MWh per year corresponds to 16.5 kg copper Thus, to increase the annual world production of solar heat to 1% of the present human energy consumption, an installation of 22 million tons of copper absorbers is required The annual production of copper worldwide is approximately 15 million tons The need for new materials is obvious Aluminum, steel, and other metallic materials will be used more Polymeric materials have to be considered as an alternative

The major advantages in using polymeric materials are low material cost in general (there also exist very expensive high-performance polymers), low weight, and low manufacturing costs The latter property is perhaps the most important factor when choosing polymeric materials for a specific application Using polymers, at least in large-scale production, complex integrated structures can be manufactured in a single step through, for example, injection molding or extrusion

The objective of the project ‘Polymeric Materials for Solar Thermal Applications’ (Task 39) of the Solar Heating and Cooling (SHC) Programme is the assessment of the applicability and the cost-reduction potential by using polymeric materials and polymer-based novel designs of suitable solar thermal systems and to promote increased confidence in the use of these products

by developing and applying appropriate methods for assessment of durability and reliability

3.12.2.3 The Collector Circuit

The durability and reliability of solar water collector systems are influenced by the behavior of the collector circuit/loop The collector circuit usually has an antifreeze–water mixture as the heat transfer fluid A heat exchanger is therefore required for heat transfer to the store Exceptions are systems that use the drain-back (Figure 14) With drain-back systems, both overheating and freezing of fluid in the solar collector loop can be protected

Efficiency of collector-types and applications

T

k: collector working temperature, (°C)

Tu: ambient temperature, (°C) G: solar irradiation, (W m–2)

Figure 13 Building-integrated solar collectors: Roof and facade

The input to the collector should always be as cold as possible, in order to keep its efficiency high Therefore, the connecting tube

to the collector is mounted at the bottom of the store, where the coldest water is

For so-called ‘high-flow’ systems with flow rate in the collector circuit of approximately 50 l h−1m−2 of collector area, the temperature rise in the collector is on the order of 10 °C The input into the store for these high-flow systems should be near the bottom of the store, and the store is heated slowly from the bottom to the top

For so-called ‘low-flow’ systems with a specific collector flow rate of 10–15 l h−1m−2 of collector area, the temperature rise in the collector is on the order of 40–50 °C The input to the store for low-flow systems should be higher up than that of the high-flow systems, the best height depending on the flow rate and system design It can be advantageous to use a stratifying unit to make sure