Volume 3 solar thermal systems components and applications 3 18 – concentrating solar power

Volume 3 solar thermal systems components and applications 3 18 – concentrating solar power Volume 3 solar thermal systems components and applications 3 18 – concentrating solar power Volume 3 solar thermal systems components and applications 3 18 – concentrating solar power Volume 3 solar thermal systems components and applications 3 18 – concentrating solar power

B Hoffschmidt, S Alexopoulos, C Rau, J Sattler, A Anthrakidis, C Boura, B O’Connor, and P Hilger, Aachen University

of Applied Sciences, Jülich, Germany

© 2012 Elsevier Ltd All rights reserved

3.18.1 Introduction

3.18.2 General Principles of Concentrating Systems

3.18.2.1 Concentration Effect

3.18.2.2 Energy and Mass Balance

3.18.2.3 Grid-Connected or Island Systems

3.18.2.4.1 Closed-circuit recooling systems

3.18.2.4.2 Wet cooling systems

3.18.2.4.3 Mechanical draft cooling systems

3.18.2.4.4 Natural draft cooling towers

3.18.2.4.5 Hybrid cooling towers

3.18.2.4.6 Fan-assisted natural draft cooling towers

3.18.4.1.1 Principles and technologies of solar cooling

3.18.4.1.2 Thermally driven cooling systems

3.18.4.1.3 Absorption chillers

3.18.4.1.4 Adsorption chillers

3.18.4.1.5 Best-practice examples for solar cooling

3.18.4.1.6 State of the art of solar cooling

3.18.4.1.7 Market expectations for solar cooling

400 600 800 1000 1200 1400 1600 1800 2000

(Tabsorber = Tprocess) (K) Flat-plate collector

Parabolic trough

η max 0.5 0.6 0.7 0.8 0.9

3.18.2 General Principles of Concentrating Systems

3.18.2.1 Concentration Effect

Theoretically, concentrating solar systems can reach considerably higher temperatures without reducing their thermal efficiencies According to Carnot’s law, this means an improved conversion efficiency of the coupled thermodynamic cycle, so that the same amount of electricity can be produced by a smaller collector area

The maximum thermal efficiency of a thermodynamic cycle is given by Carnot’s law:

TP −T

th ; Carnot ¼

TP

where TP and TA are the process and ambient temperatures, respectively

The upper boundary of the efficiency of solar thermal power plants is given by

ηmax ¼ ηth ; Carnot ⋅ηabsorber

Figure 1 shows the theoretical possible achievable efficiency as a function of the absorber temperature For simplicity, the absorber temperature is set equal to the process temperature In reality, the process temperature is less due to losses As can be seen from the

Figure 1 Upper boundary of solar thermal power plant efficiencies

Another very useful dimension that characterizes solar thermal systems is the concentration ratio C It is defined as the ratio of the collector aperture area to the receiver area The whole collector field has to be considered

collector aperture area

C ¼

receiver area Flat-plate collectors have a concentration ratio of 1, parabolic trough and Fresnel collectors about 100, solar towers up to or more than 1000, and solar dishes around 4000 For example, for the Solar Tower Plant of Jülich (STJ; see Section 3.18.5.1.4) in Germany with a receiver area of more than 20 m2 and a heliostat field area of approximately 18 000 m2, a concentration ratio of around 900 has been reached

3.18.2.2 Energy and Mass Balance

In order to thermally analyze a solar thermal concentrating system, the mass and energy conservation law has to be considered for the whole system as well as for each component Especially, the components of the solar cycle have to be considered in detail The thermal efficiency of solar thermal absorbers is given by the following equation:

εσsT4

ηabsorber ¼ αeff −

CS where ε is the emission coefficient, αeff the effective absorptivity, σ the Stefan–Boltzmann constant, and S the solar input The concentrated solar energy received by an absorber is absorbed to a large degree, but losses also occur The losses at the receiver may

be due to radiation, convection, and conduction, as well as thermal losses occur during the transport of heat transfer fluid (HTF)

A detailed energetic consideration of each solar absorber technology can be viewed in Chapter 3.06

3.18.2.3 Grid-Connected or Island Systems

Solar thermal concentrated power systems can be connected in grid as well as in island systems For small island systems, solar dish systems are a suitable solution Parabolic trough collectors (PTCs), Fresnel collectors, and solar towers may be grid or island systems Together with a thermal storage or after hybridization, dispatchable power can be provided

3.18.2.4 Recooling

In a condensation power station, a main condenser or a turbine condenser, in which the steam flowing away from the turbine condenses out, can be used Condensation heat must be led away from the system, to the ambient This is done by the circulation of different coolants Using different systems, heat is delivered either to the hydrosphere (heat removal directly by the main condenser)

or to the atmosphere (heat removal indirectly by air-cooled heat exchangers)

3.18.2.4.1 Closed-circuit recooling systems

The closed-circuit recooling system is thermodynamically closed By means of a finned tube bundle heat exchanger, the medium on the product side is cooled (Figure 2)

The use of finned tubes is necessary, because based on the low heat transfer coefficient of the air, a high air mass flow is necessary The large need for air can be avoided by enlarging the air-side heat exchange surface compared with the surface on the coolant side The big disadvantage of closed-circuit recoolers is the coolant temperature reached, which is close to ambient temperature This affects the energy efficiency of the power station

3.18.2.4.2 Wet cooling systems

Thermodynamically, this system is described as an open process The warm coolant is injected into the cooling system and then conducted through a special fill material The loss of heat occurs by means of convection and mass transport (Figure 3)

Figure 2 Closed-circuit recooling system

Cooling system

System

Air Figure 3 Wet cooling system

In such a system, lower temperatures occur and smaller transfer surfaces can be reached compared with a system using the circuit coolers due to cooling to the wet-bulb temperature Disadvantages of wet coolers are vapor production and the demand for additional cooling water

closed-3.18.2.4.3 Mechanical draft cooling systems

As for air supply, there are two different cooling tower variations, supplying cooling air either by forced or by induced draft to the system Both variations are used in the closed-circuit as well as in the wet recooling system Figure 4 shows different variations of arrangement

• Forced draft A mechanical draft recooling system with a blower-type fan at the intake The fan forces air into the system A forced draft design typically requires more power than an equivalent induced draft design In combination with a wet recooling system, the fan on the intake of the cooling tower is more susceptible to complications due to freezing conditions

• Induced draft A mechanical draft recooling system with a fan at the discharge, which pulls air through the tower The fan induces hot and wet air in combination with a wet cooling system

3.18.2.4.4 Natural draft cooling towers

In the natural draft cooling tower, the necessary air mass flow is caused by density differences (buoyancy) Figure 5 shows the function

of a natural draft cooling tower with closed- and open-circuit cooling systems The heat exchange surfaces are right in the lower part of

Hot water Hot water

Cold water Cold water

Fill

Hot water

Cold water

Centrifugal fan

Cold water Sump

Hot water distribution Louvers

Heat exchanger Air out

Coolant in

Coolant out

Convection Buoyancy

Convection and mass transfer

Collection basin

Warm moist air

Coolant in

Air inAir in

Fill material

Coolant out Figure 5 Natural draft cooling towers: closed circuit (left) and open circuit (right)

the tower, producing current by buoyancy Compared with the mechanical draft system, the advantage is that the natural draft cooling tower does not demand power for the fans The result is a positive impact on the achieved balance of the whole power station The natural draft wet cooling tower works in a similar way as the natural draft cooling tower with closed-circuit cooling system, but instead of the heat exchanger fill material is installed and the heat transfer mechanism functions in a different way

3.18.2.4.5 Hybrid cooling towers

The hybrid cooling tower is a combination of a mechanical draft closed-circuit system and a wet recooling system The warm coolant

is partly injected into the cooling system; the other part is cooled down by an air-driven heat exchanger and can afterward be injected into the cooling system (Figure 6)

Fan Diffuser

Mixers Heat exchanger Noise attenuator Drift eliminator

Fill Rain zone Noise attenuator Water basin

Figure 6 Hybrid cooling tower Source: http://www.wetcooling.com

The cooling tower can be operated according to the change in ambient temperature At a low-level ambient temperature, only the air driven by heat exchanger is in operation In case the ambient temperature increases, the wet cooling system can also be activated,

to reach a very low coolant temperature Hence, problems that occur in winter in the wet cooling systems can be avoided and in summer very low coolant temperatures can be reached Moreover, the heat exchanger decreases fogging caused by the wet cooling system When applying the hybrid driving mode, coolant consumption is considerably reduced

3.18.2.4.6 Fan-assisted natural draft cooling towers

The design is similar to the natural draft cooling towers (Figure 7) In addition, big fans are introduced in the lower cooling tower This additional air mass flow needs a smaller cooling tower However, the mass flow caused by buoyancy is still very high, so that the fans are driven with low speed

To fix this problem, the air-driven condenser can be combined with a hybrid cooling tower and a conventional surface condenser which transports the condensation heat to cooling water (Figure 9) This, however, requires higher capital costs Thus, lower pressure in the condenser can be reached even if the ambient temperature is high One such installed air-driven condenser, named LUKO of GEA Energietechnik GmbH, is shown in Figure 10

3.18.3 Power Conversion Systems

3.18.3.1 Solar Only

3.18.3.1.1 Steam cycles

At present, most solar thermal power plants convert solar power into electricity applying Clausius–Rankine cycles These steam cycles are basically of conventional technology and have only to be adjusted to the needs of the solar system For example, the steam generator works with an HTF like thermal oil for the evaporation of water When integrated with a solar cycle, some adjustments on the conventional cycle have to be made mainly in the heat recovery steam generator (HRSG), in order to consider the dynamical behavior of the solar part due to the changing weather conditions

A steam cycle can be attached to almost all solar thermal systems except the solar dishes Figure 11 shows the operational scheme for a parabolic trough system with thermal oil as an HTF in combination with a conventional steam cycle

Figure 12 shows the molten salt tower system with components of a conventional power plant, such as a steam generator Also a Fresnel system can be combined with the steam cycle of a conventional power plant

Depending on the dimension of the power systems, one, two, or three pressure steam cycles in the steam are used Taking into consideration the HTF used, one-cycle and two-cycle systems are applicable in order to transport the heat effectively to the steam turbine

The main advantages of the conventional steam cycle are as follows:

Figure 7 Fan-assisted cooling tower Source: GEA Energietechnik GmbH

Windwall

Counterflow module

Parallel-flow module Forced draft fan

Condensate tank Figure 8 Two-stage, single-pressure air-driven dispenser Source: GEA Energietechnik GmbH

Figure 9 Parallel Condensing system (PAC®

) Source: GEA Energietechnik GmbH

Figure 10 Installed air-driven condenser Source: GEA Energietechnik GmbH

EPGS

Figure 11 Process flow diagram of parabolic collector field combined with a steam power cycle

Figure 12 Process flow diagram of molten salt tower plant EPGS, electric power generating system Source: Sandia National Laboratories

• Reasonably high efficiency of the steam cycle, especially the steam turbine

• High scale of power capacity from 1 MW to more than 1000 MW

• Applicable for the temperature ranges of solar thermal power plants

• Can be combined with hybrid systems as bottoming cycle to achieve higher efficiencies

The disadvantages are as follows:

• High water consumption for the cycle and/or for recooling

• Personnel need to have knowledge of the complex systems

• Can hardly be used as remote automatic power plant

• Not applicable for small-scale power plants

High efficiency is reached for large-dimension power plants due to the fact that for these plant sizes the components of the steam cycles are state of the art But with appropriate adjustments to the conventional parts, even for small power plants good thermal efficiencies can be reached, and the high investment of the collector field is compensated

For the efficiency of a Clausius–Rankine cycle, recooling is important, because the backpressure of the steam turbine reduces the possible expansion of the steam from high pressure to low pressure (vacuum) The backpressure is dependent on the recooling temperature As solar power plants with recooling are located in regions with high ambient temperatures, this may have an impact when using recooling systems that use ambient air

3.18.3.1.2 Organic Rankine cycles

Another alternative to conventional Clausius–Rankine cycles is the organic Rankine cycles (ORCs) used to convert thermal energy into electric energy They work in a similar way but use an organic fluid for the cycle These cycles are applied to lower temperature heat sources compared with water/steam cycles ORCs are mainly used in combination with geothermal power or waste heat recovery in industrial processes The range of the electrical power output is from some kilowatts to about 10 MW Additionally, thermal energy can be produced, which does increase the overall efficiency Due to the low temperatures, ORCs possess a lower theoretical Carnot efficiency at a maximum of 30% [1] than water/steam cycles The temperatures of the working fluids are limited

to about 250–350 °C due to thermal stability thresholds [1] Working fluids include organic (hydrocarbon) fluids, namely, R-123, R-134a, R-245fa, R-717 (ammonia), R-601 (n-pentane), R-601a (isopentane), and C6H6 (benzene)

Generally, an ORC consists of components that are also included in a Clausius–Rankine cycle The cycle consists of an evaporator, a superheater, a turbine/expander, a condenser behind the turbine, and a pump Because of the organic fluids and their different freezing points, usually lower than that of water, the condenser does not work in subatmospheric pressures and is water- or air-cooled In this configuration, ORCs with upper cycle temperatures of about 300 °C have an efficiency of about 12.5% [2] The efficiency of the basic Rankine cycle can be increased by recuperation and reheating, which lead to efficiencies of 20.5% for the same boundary conditions [2] In this case, the expanded fluid at the second turbine stage heats the fluid before entering the evaporator and is itself cooled down Reheating takes place after the expansion in the first turbine stage by redirecting the heat to the boiler

Additionally, when higher thermal energy source temperatures are available, the combination of two or three ORCs using different working fluids with different saturation temperatures can increase the overall cycle efficiency

Reasonably high cycle efficiencies compared with other energy conversion techniques can be obtained by an ORC Herein, the high turbine efficiency (up to 85% [1]) is the main advantage Due to the low turbine speed, low mechanical stress, and absence of moisture in the vapor nozzles (thus no erosion of the blades), a long lifetime is guaranteed Furthermore, the ORC has less operation and maintenance (O&M) costs and can be operated remotely because of the low pressures compared with steam cycles Simple start–stop procedures and good part load behavior make the ORC an attractive energy conversion system

Despite these advantages, the negative aspects have to be mentioned The limited overall efficiency and the limited plant size due

to the low source temperatures allow ORCs to be used only for certain applications Using organic fluids may not be an environmentally friendly option, and some of these are forbidden by the Montreal Protocol as they are capable of destroying the ozone layer Furthermore, dykes have to be installed to hinder fluid from entering the ground

ORCs can work at lower temperatures, so the operating temperature of the parabolic trough can be reduced from about 400 to 300 °C This allows to use inexpensive HTFs (e.g., Caloria) and combine it with a two-tank thermal energy storage system Furthermore, lower operating temperatures result in lower capital cost for the solar components By using organic fluids for the power cycle instead of water in combination with air cooling of the power cycle, the use of water is reduced to only the amount for cleaning the mirror surfaces, which is about 1.5% of the total water use at the solar electric generating systems (SEGSs) [2] Another advantage, as mentioned before, is that the plant operation can be done remotely This further reduces the O&M costs of solar thermal plants

The combination of ORC and solar collectors follows a trade-off (Figure 13) between the single efficiencies to gain the maximum overall system efficiency Increasing the temperature of the process will lead to a lower collector efficiency, but increases the ORC efficiency Choosing the right medium temperature has a great impact on the overall system efficiency

ORC installations have increased in the last few decades because of the optimization of the efficiency of industrial processes and the intensive application of renewable energies like biomass and geothermal At present, the share of different applications in the ORC market is as follows: 48% biomass, 31% geothermal, 20% waste heat recovery, and 1% solar [3] The solar thermal share of the ORC market is still low, but some applications have been installed A choice of these installations is listed below:

η

Collector efficiency ORC efficiency

Solar Receiver Combinedunit cycle plant

Gas

Heliostat field

Gas turbine Steam cycle

Figure 14 Solar ORC in Lesotho Source: Solar Turbine Group

• Arizona Public Service (APS) built a 1 MWe concentrated solar power (CSP) plant working with ORC in Arizona in 2006 Ormat International has supplied the 1.35 MWe gross power block and Solargenix Energy Inc was the system integrator and the vendor

of the 10 340 m2 parabolic trough field [4] The ORC runs with n-pentane as the working fluid and the peak solar-to-electrical efficiency is about 12.1% at the design point The annual solar-to-electrical efficiency is 7.5% [5]

• Another prototype plant was built by GMK in Germany in 2005 The ORC power output is 250 kWe, with the electrical efficiency

of the system being about 15% A parabolic trough field has not been built; instead, the solar system has been simulated by a natural gas boiler [3]

• The Solar Turbine Group developed small-scale systems for remote off-grid applications Figure 14 shows a 1 kWe system installed at Lesotho Such systems have been developed to replace diesel generators in off-grid areas of developing countries

By using materials available in this country and designing an ORC that runs at medium temperatures, the levelized electricity cost (LEC) can be lowered (~0.12 $ kWh−1 compared with ~0.30 $ kWh−1 for diesel) [3]

3.18.3.1.3 Gas turbines

Introducing concentrated solar energy into a gas turbine (GT) system implements a new GT power plant concept The GT can be combined with a dish/Brayton system as well as a solar tower The receiver considered in both cases is a pressurized closed volumetric one The heat transfer medium is forced through the receiver structure and is heated by convection

In the combination of the GT with a dish/Brayton system, the solar radiation is focused by the dish concentrator and it enters the receiver through a quartz window The window guarantees the closing of a pressure vessel Air as an HTF enters the combustor and passes through the receiver, where it is heated up

If required, an additional burner enables the daily operation of the plant Hot air enters the turbine where it expands; the exiting hot air is connected to the GT combustor and the cycle is closed

When the GT is combined with a solar tower (Figure 15), the receiver absorbs the concentrated solar irradiation and transfers the solar heat to pressurized air, which can be heated up to 1000 °C The hot pressurized air from the solar receiver is directly fed into the combustion chamber of a GT, where natural gas is added to further heat the air to the turbine firing temperature design point

Figure 15 Scheme of solar hybrid GT system Source: DLR

Solar receiver and combustor

Concentrated sunlight

Parabolic dish concentrator

The serial connection of pressurized solar air receivers with the combustion chamber of a GT allows a solar hybrid operation that can compensate for any deficiency in solar radiation Thus, the power output of the solar-hybrid power plant can be guaranteed, independent of the sun’s position or meteorological conditions, and still meets the utilities power demand requirements Depending on the system configuration and operation strategy, the power output from solar energy can be between 40% and 90%, depending on the design conditions [6]

The concept of solar hybrid GT systems leads to the following advantages:

• High cost reduction potential due to the high conversion efficiency

• Low environmental impact due to low water consumption

• GT Brayton cycle – no cooling water required

• combined cycle configuration – up to 70% less cooling water required

• Reduced land use due to high conversion efficiency, which reduces collector area and land use

• Guaranteed dispatchable power

Different international projects like REFOS and SOLGATE have already been completed Some are currently ongoing, like Solugas [7, 8]

The main objective of the Solugas project is to demonstrate the performance and cost reduction potential of a solar hybrid-driven

GT system on a commercial scale The German Aerospace Centre (DLR) and industrial partners, like GT manufacturer Turbomach and Abengoa Solar, participate

3.18.3.1.4 Solar dishes

A dish/Stirling power plant is made of a parabolic dish concentrator, a solar receiver, and a Stirling engine The system is hybrid, as it uses fuel to supplement solar power Figure 16 demonstrates the operational scheme of such a system When solar energy is unavailable, the system can operate with fuel alone Some dish/Stirling units have been tested and show tens or thousands of operation hours

The Stirling engine can be replaced by a recuperated GT In that case, a dish/Brayton system is formed as seen in Figure 17 Many solar dishes, including Stirling or Brayton, can be combined together to form a solar dish power plant

3.18.3.2 Increase in Operational Hours

Many hybridization options are possible with natural gas combined cycle and coal-fired or oil-fired Rankine plants

Figure 16 Operational scheme of a dish/Sterling system Source: Pitz-Paal R, Dersch J, and Milow B (2005) European concentrated solar thermal road-mapping SES6-CT-2003-502578 [9]

Concentrated sunlight Compressor

Parabolic dish concentrator

3.18.3.2.1(i) Integrated solar combined cycle system

A hybrid solar power station is a solar thermal power station that uses a second energy source for heat production, besides the solar radiation Non-fossil fuels, hydrogen, methanol, or fermentation gas can be used These fuels are used when more electric energy is needed in case of low solar radiation In addition, thermal energy can be stored to buffer the heat flows temporarily The integrated solar combined cycle system (ISCCS; Figure 18) combines a solar parabolic through-collector field with a combined cycle power station The heat recovery boiler is modified to enable additional steam production by a solar steam generator or an afterburner Solar heat is only partially used for steam production; in the absence of storage technologies, the annual solar heat produced in such types

of power station is less than 20% (Figure 19)

Steam turbine Fuel

Flue gas

Condenser

recovery system High-pressure steam Solar steam

vessel

preheaterDeaerator

Figure 18 ISCCS schematic diagram Source: Flagsol

Figure 19 (a) Open-load and (b) base-load curves of ISCCS

3.18.3.2.1(i)(a) Preheating for fossil power plants A solar thermal system may be a heat source, providing necessary heat for a heat exchanger of a steam power cycle, which might be a water preheater On sunny days, the solar concentrated system provides the necessary heat; otherwise, fossil fuels in the conventional power plants take over For preheating applications, Fresnel collectors, parabolic troughs, and solar towers are suitable The collector area is specially dimensioned in order to match the required energy input It may be located next to an existing coal or lignite power plant Integration of a solar field enables reduction of CO2 emission and saves fossil fuel

3.18.3.2.1(i)(b) Gas engine Gas engines also enable hybridization of solar thermal power plants Compared with other engines, gas engines may be of higher efficiency and there will be less environmental impact when using natural gas and biogas A gas engine can work with biogas of low quality, which would be more difficult for GTs Instead, GTs permit a higher variety in CH4/CO2 mixture The advantages of using biogas compared with other biomass fuels are mainly the biomass-to-land use ratio due to the application of the whole plant, transport of biogas via pipeline, and the utilization in various machines and applications Also hybridization of solar thermal power plants with biogas is of interest as the status of a renewable energy conversion plant is maintained

Gas engines can be combined with parabolic trough as well as with tower power plants Lower temperatures (in the range of

300 °C to almost 400 °C) of the flue gases of gas engines compared with those of GTs have to be considered Due to this fact, gas engines are more applicable for parabolic troughs using thermo fluid The flue gases of gas engines can also be integrated into other solar thermal plants, depending on their concept In power plants running steam temperatures higher than 400 °C, the gas engine flue gases might be used for preheating Compared with GTs, gas engines have a lower volume flow-to-installed capacity ratio because of almost stoichiometric combustion Therefore, and due to efficiency of about 40% and more, the installed capacity of gas engines is higher to achieve a certain thermal power output This capacity can be reached by using one or more engines that are connected Possibly engines of capacities up to 100 MWe may be installed to power parabolic trough power plants of 50 MWe More engines with less individual capacity can permit a better controllability of part load Instead, in GTs, the combustion is over-stoichiometric, and additionally air is soaked in for cooling the turbine blades

The use of gas engines for solar thermal power tower plants or Fresnel collectors with direct steam generation (DSG) depends on the live steam temperatures and the flue gas temperature of the engine For DSG and power tower plants with water tube receiver and saturated steam production, the live steam parameters may be at about 400 °C where certain gas engines can produce flue gases

at these temperatures Nevertheless, even if the temperatures are too low for live steam production, gas engine flue gases can be heated up by an additional burner or can be used for preheating or evaporation The flue gases’ energy can be transmitted by a heat exchanger to the heat transport media or directly to the water/steam cycle In combination with the open volumetric receiver concept of solar tower plants, the flue gases can be mixed with hot air which is heated up by the receiver and enter the HRSG The hybridization of solar thermal power plants with gas engines predicts a dispatchable power plant which is independent of thermal storage, although the plant may include one for solar-only operation At present, such hybridized plants speed up the market introduction of solar thermal power plants

3.18.3.2.1(i)(c) Burner Hybridization is also possible with an additional burner in order to provide the steam parameters at the inlet of the steam turbine

At the SEGS power plants, the hybridization with a burner was realized Figure 20 shows the schematic diagram of an optimal deployment of a burner in a parabolic trough system

Thermal oil as an HTF is transported through the collector field and heated up The heat is transported to water, and steam is produced in a boiler and expanded in the steam turbine, producing electricity An additional burner provides the steam required on days with less solar radiation or at night

The same hybridization may be realized when water/steam is used as an HTF This concept has the advantage of leaving out heat exchangers, and is demonstrated in Figure 21

Figure 20 Additional burner for a PTC power plant G, generator

Figure 21 Additional burner in a DSG solar power plant G, generator

To achieve the gas parameters at the inlet of the HRSG, at a solar tower system with an open volumetric receiver, for nominal load even by less solar radiation, a channel burner is included Figure 22 shows the realization for such a hybrid system The burner heats up the preheated air from an open volumetric receiver to the desired temperature [11] It can be operated in parallel or instead of the solar air receiver Parallel means that on a day with little solar radiation, both components can provide the heat for the HRSG at the same time In the other operation mode, the hybrid component is only switched on in the night or at times

of no solar radiation

3.18.3.2.1(i)(d) Gas turbine A GT can be combined with a solar tower with the use of open volumetric receiver In the position

of the burner, as shown in Figure 22, a GT is placed The GT is in a position parallel to the receiver The exhaust gas from the GT is mixed with the air from the receiver to get the nominal mass flow at a high temperature After the HRSG, the exhaust gas is

Open volumetric receiver

Heliostat field

Air recirculation Cold air

recirculated to the receiver or can be passed to a stack This hybridization concept has the advantages of combined cycles, like high efficiency, and additional power production by the GT As fuel, natural gas or biogas (to maintain the status of the plant using renewable energies) can be used In addition, the hybridization concepts can be used in different operation modes

The combination of GT and closed volumetric receiver type for solar tower plants also permits hybridization The compressed air

is heated up by the concentrated irradiation and directed to the turbine In case the solar power cannot heat the air to the needed turbine inlet temperature, a burner can be used to achieve the temperature by additional firing This plant can also be operated in case of no solar radiation by using this setup as a conventional GT power plant

3.18.3.2.2 Storage

Storages are classified as long- or short-term storages, according to their purpose

While long-term storages are used to compensate for seasonal temperature fluctuations, short-term storages are intended for periods from a few minutes up to several days Depending on the energy to be stored, there are a multitude of storage possibilities Storage of electrical energy is either difficult to achieve or very expensive The classic long-term storages in the energy sector are pumped storage plants

A thermal storage system for solar power plants is used as a buffer It stores thermal energy in times of high irradiation and enables the operation of the power plant after sunset and during periods of reduced solar input or cloud passages By using storages, solar thermal power plants also gain the following advantages:

• Conservation of the conventional power plant components (protection of the turbine against increased loading due to constant starting up and shutting down)

• Cost savings for the power plant components – for example, due to the components of the steam cycle not having to be laid out for the maximum power rating of the receiver – which results in increased hours of operation of the steam cycle

• Possibility of various start-up strategies

• Enabling the supply of solar electricity at especially profitable times

• Flexibility – due to the storage and steam cycle having the same working temperature, the energy can usually be drawn faster from the storage and be directly supplied to the steam cycle; thus, warm-up times and delays in the process cycle are avoided

A great advantage of thermal power plants is the ability to store energy in many relatively low-priced ways Economical thermal storage is a key technological issue for the future success of solar thermal technologies

3.18.3.2.2(i) Solid media

The classic method of storing thermal energy is to choose a solid storage medium and increase its temperature by sensible heat transfer Thermal storages with hot gases flowing through them have been in industrial operation for many years These so-called regenerators are employed in flue gas cleaning and heat recovery in the metal industry

This principle, as described in Reference 12, is implemented in the Jülich solar tower and serves as a heat storage for the solar tower power plant with an open volumetric receiver The principle is depicted in the schematic diagram shown in Figure 40 During charging, the HTF flows through the storage medium (Figure 23) For unloading, the flow direction is reversed

A temperature profile may develop over the height of the storage This principle is illustrated in Figure 24 The steeper the temperature gradient, the greater the amount of storage medium that can be used for storage The storage capacity varies mainly depending on the storage material chosen In addition, the shape and distribution of the inventory materials are the decisive factors for the actual storage capacity per storage volume

As mentioned in Reference 13, packed bed designs appear particularly attractive, but they have additional technical risks stemming from mechanical loads occurring during cyclic operations A concrete heat storage developed by DLR offers low-cost prerequisites for heat storage for parabolic trough power plants with thermo oil as an HTF [14]

3.18.3.2.2(ii) Organic media

The term ‘organic heat transfer media’ describes hydrocarbon compounds as they are used in the ORC processes Thermo oils used

in current parabolic trough power plants also belong to this media group Classical methods of heat storage for single-phase HTF are the one- and two-tank storage systems [15–17]

In the one-tank thermocline storage system, a single tank is used, which contains both the hot and cold heat transfer media

In order to prevent mixing of hot and cold fluids in the container, a thermocline stratification must be maintained During charging

of the storage, cold fluid is taken from the bottom of the tank, and after heating it flows back into the tank from the top During discharge, the flow direction is reversed A simultaneous charging and discharging is theoretically possible if there are two sets of hot and cold pipes In the two-tank system (hot/cold tank configuration), the hot and cold media are stored in separate tanks

In this way, a mixing, and therefore an exergy decrease, is prevented This advantage, however, is achieved only with additional costs because of the need for two tanks

3.18.3.2.2(iii) Phase-change materials

Due to their phase-change enthalpy, phase-change materials (PCMs) offer the greatest potential for large storage capacities per storage volume In addition, during the entire melting time, this process provides an almost constant source temperature [18] These materials have great advantages for operation with dual-phase HTFs This technology is not as widespread as that of the sensitive heat storages

Storage chamber

Cross-sectional area

of honeycomb elements

Honeycomb storage elements

Storage temperature profile

Storage capacity 100%

Storage capacity 0%

Storage height Figure 24 Temperature profile of the storage for 100% and 0% storage load

(SHSs) The disadvantage is the usually lower heat transfer coefficient that can develop during loading and unloading The reason is the solid–liquid interface, which moves away from the heat transfer surface during loading, causing an increase in the thermal resistance A combination of PCM and SHS systems is the focus of current research [19]

There is an ongoing attempt to combine both advantages Good heat transfer coefficients can be achieved with the SHS technology, and a high storage capacity can be reached using the latent heat storage (LHS) technology

First, advanced storage materials based on PCM technology adapted to steam generation/condensation in the range of

200–300 °C have been tested by DLR in Almeria, Spain, in a plant with a power rating of 100 kW

An expansion to 1000 kW with operating temperatures of 300 °C is being sought as described in Reference 19

3.18.3.2.2(iv) Adiabatic pressurized air storage

Adiabatic pressurized air storage shows a new and low-cost possibility to store huge amounts of energy [20, 21] This technology combines pressurized air storage with a heat exchanger A standard method is to store pressurized air in caverns A schematic diagram is shown in Figure 25

During the charging process, ambient air is first compressed under high pressure and then in a subsequent step it passes the heat

to a thermal storage system When discharging, the stored heat is used The pressurized air leaves the cavern and flows through the storage where it is heated up Then the hot pressurized air is passed to a GT and electricity is produced By this process, no additional

3.18.3.2.2(v) Others

A high cost factor for all types of storages is the cost of the inventory materials Considering future locations of solar power plants, raw sand which is almost free of charge would be an interesting storage medium The concept of sand as a storage medium for a solar tower with a volumetric receiver has been evaluated to have high potential by DLR in Cologne (Figure 26) While in a ceramic storage one-third of the costs incurred are for the storage material, sand would be nearly free of charge and sufficiently available [13] The key component is the air–sand heat exchanger In recent years, a concept has been developed in the Solar-Institut Jülich (SIJ), and a test rig has been constructed [12] In the cross-flow heat exchanger, sand flow is separated from the airflow channels by ceramic and woven-metal filter walls The air flows through the sand bed, thus generating a temperature profile

The sand which is heated up in the air–sand heat exchanger to approximately 800 °C (not directly irradiated as with a particle receiver [23]) flows to the hot storage through a downpipe and further to the fluid bed cooler The fluid bed cooler, a unit which is a standard component of fluidized bed combustion units, is the driving element of the steam cycle The generated steam is finally fed

to the steam turbine for power generation The cooled sand exits from the fluid bed cooler at a temperature of approximately 150 °C and either returns to the air–sand heat exchanger or is stored in the cold storage tank [12]

Air−sand heat exchanger

Cold storage

Tower

Hot storage

Turbine

Figure 26 Sand storage concept for solar power towers Source: German Aerospace Centre (DLR), SIJ

Electric systems

Solar cooling Photovoltaic effect/

Photovoltaic module:

(vapor compression cycle)

Thermally driven systems

Water/ammonia

Dehumidifier rotor

Solid sorbent

Water/silica gel Water/zeolite

Dry absorption

solar energy is feasible without the use of ozone-depleting chlorofluorocarbons (CFCs) and helps lower the emission of greenhouse gases (GHGs) The global cooling demand is steadily increasing and varying The potential in Europe for solar heating and cooling is presented in Reference 24

3.18.4.1.1 Principles and technologies of solar cooling

The driving heat for any thermally driven chiller can be produced by solar energy with the appropriate technology, concentrated or not concentrated, that allows different temperature levels for the inlet heat Varying cooling demand and intensity of solar radiation show the same pattern at many geographical sites which can be regarded as a benefiting coincidence Another benefit is the fact that the use factor of the solar appliances (collectors, concentrators, and complete plants) can be easily increased when cogenerating heat and cold The variation in principles is huge Figure 27 is an overview of different groups of solar cooling principles

3.18.4.1.2 Thermally driven cooling systems

Well-proven thermally driven cooling technologies for cold production are absorption and adsorption chillers Absorption cooling uses a liquid sorption material or a solid material (dry absorption) Solar thermal energy changes the refrigerant into a vapor Adsorption cooling uses a solid sorption material (water/silica gel or water/zeolite) Solar thermal energy dries out or regenerates the desiccant The process is called the desiccant cycle The cycles can be both continuous and intermittent

3.18.4.1.3 Absorption chillers

In absorption chillers, a thermochemical process is used to drive a vapor compression cycle The refrigerant in an absorption chiller dissolves in an absorbent solution for which it has a high chemical affinity Two common refrigerant/absorbent combinations are water and lithium bromide (LiBr–H2O) (used in water-cooled absorption systems) and ammonia and water (NH3–H2O) (used in air-cooled systems) Thermal energy for the absorption process can be supplied in different ways, for example, by gas burners and by recovering thermal energy from concentrated solar thermal power plants (cogeneration) The number of heat exchangers within the absorption chiller distinguishes the system as either single-effect or double-effect absorption chiller Single-effect absorption chiller systems consist of an evaporator, an absorber, a generator, a separator, and a condenser and is shown in Figure 28

The evaporator generates chilled water at 4–10 °C that is pumped to air conditioning units in the air distribution system Solar flat-plate, evacuated tube, or parabolic trough collectors can be used to heat water from 77 to 99 °C, which is circulated through the generator to heat the refrigerant/lithium bromide solution

Double-effect absorption chiller systems use a high-temperature generator and a low-temperature generator to improve the thermodynamic efficiency of the absorption cooling cycle Operation of the evaporator and the absorber is the same as that of the single-effect system The high temperature for the heating cycle of a two-stage chiller requires concentrating solar collectors, like PTCs Steam units operate with a pressure of 3.5–9 bar, which can be produced directly by a field of trough collectors In triple-effect absorption chiller systems, two single-effect absorption circuits are combined A basic figure to describe the quality of the conversion of heat into cold is the thermal coefficient of performance (COP) It is defined as the useful cold (Qcold) per unit of invested driving heat (Qheat) Single-effect lithium bromide chillers operate with thermal COP that is limited to about 0.7 Double-effect chillers can have thermal COP in the range of 1.0–1.5

Figure 27 Overview of different processes of solar cooling

Vapor refrigerant

Chilled water Liquid refrigerant

Cooling water Concentrated lithium bromide solution

Thermal energy Dilute lithium bromide/refrigerant solution

Heat medium (solar or gas)

Lift pipe Separator

Refrigerant vapor Condenser Orifice Evaporator Chilled water

Absorber Cooling water

Heat exchanger Generator

Figure 28 Single-effect absorption chiller Source: http://www.yazakienergy.com

3.18.4.1.4 Adsorption chillers

The adsorption cycle needs a lower heat source temperature than the absorption cycle It operates with silica gel/water

or with zeolite/water as working pairs Adsorption and desorption take place in reversible cycles Large heat exchange surfaces permit a mode of operation close to the thermodynamic limits An adsorption chiller consists of two or four chambers, a condenser, and an evaporator Figure 29 shows a simplified schematic of an adsorption chiller with two chambers

In the system shown in the figure, the water, bounded to the silica gel, is driven out of the right chamber while heat is being transferred into the same stage Subsequently, the water is liquefied in the condenser The heat is transferred to the cooling water In the low heat chamber (left), the water vapor is adsorbed and the resulting heat is dissipated to the cooling water The inlet temperature of the hot water side of an adsorption chiller can be below 75 °C Thus, solar or waste heat of a low temperature level can drive an adsorption chiller [25]

3.18.4.1.5 Best-practice examples for solar cooling

The increasing demand for small-scale solar heating and cooling systems results in a number of installed systems for air condition­ing in residential houses or small offices Within IEA Task 38, a large number of these systems are included in the monitoring and evaluation procedures [26] To date, 23 systems have been identified which cover all relevant technologies and a broad range of different climatic conditions An overview of installed solar cooling systems worldwide is given in Reference 27 The AEE Institute for Sustainable Technologies (Austria) runs a project with several partners to demonstrate industrial cooling technology using a solar-driven steam jet ejector chiller (SJEC) The steam uses water as a refrigerant An operation between 150 and 200 °C is planned Therefore, PTCs are being coupled

Figure 29 Schematic of an adsorption chiller 1, Chamber 1; 2, chamber 2

T H

400

Chilled waterCOP / ξCarnot system (e.g.,

350

fan coil),1.1 / 0.4 Chilled water

air-cooledsystem (e.g.,

200 cooling (e.g., chilled ceiling)

Useful temperature lift ΔT = TM − TC (K)

Solar field Storage Solar field Storage Solar field Storage

Figure 30 Required heat source temperature as a function of the required temperature lift between required temperature of cold production

(low-temperature heat source) and temperature for heat rejection Curves refer to different COP/ξCarnot ratios (ξCarnot is the Carnot efficiency factor) Typical operation temperature ranges of different solar collector technologies are marked by colored areas The ellipses indicate different system designs [28]

3.18.4.1.6 State of the art of solar cooling

Some of the cooling technologies described are still in the status of pilot projects or system testing Figure 30 gives an overview, which compares the different systems with respect to different driving temperatures according to temperature lift showing the suitable combination of solar collector and cooling technology with their expected COP

3.18.4.1.7 Market expectations for solar cooling

The growing demand for air conditioning in homes and small office buildings is producing a growing market for different types and scales of solar-assisted cooling technologies Basic and applied research in the field of thermally driven cooling is currently dominated by Japan and, increasingly, China Europe lagged behind for some time, but due to advances in the efficiency of energy transformation chains, European R&D institutes and universities have been increasing their efforts on thermally driven cooling technology, and specifically on its operation in combination with solar energy Today, Europe is one of the global leaders in the implementation of solar thermal cooling technology [29]

Worldwide, a large number of different desalination technologies are available and applied [30] Only some of these technologies reached a semicommercial state and can be applied in large units in order to be effectively combined with CSP plants

At present, either multieffect desalination (MED) systems or reverse osmosis (RO) units are used The MED system needs electricity and heat, whereas RO needs only electricity

MED is a thermal distillation process and has gained attention due to its better thermal performance [31] MED plants can be configured for high- or low-temperature operation and can be coupled to a condenser of a steam cycle The MED process is composed of a number of elements, which are called effects As described in Reference 32, the steam from one effect is used in order

to heat another effect which, while condensing, causes evaporation of a part of the salty solution The produced steam goes through the next effect, where the same condensing and evaporating phenomena take place

RO is a membrane separation process that recovers water from a saline solution pressurized to a point greater than the osmotic pressure of the solution Membrane layers hold back the salt ions from the pressurized solution, allowing only the water to pass Figure 31 shows three different desalination systems combined with a CSP The first desalination system in combination with a solar tower (see Figure 31, left) uses only the heat from the solar cycle for the supply of the MED system with heat and the electricity

is provided by the grid In the second system, the solar tower provides only the electricity necessary for the RO unit (see Figure 31, middle) The RO plant uses electricity generated by a solar thermal power block to work the pumps The only high energy requirement is to pump the feedwater at a pressure above the osmotic pressure High pressures must be used, typically

50–80 bar, in order to have a sufficient amount of water pass through a unit area of membrane [33]

The third MED system (see Figure 31, right) includes the conventional cycle for supplying the desalination system with heat and electricity

These three systems may find application in islands and coastal areas that face water shortage problems

Desalination plants are to be operated nonstop Therefore, a solar tower power plant should be in operation day and night [34] This is possible only with an additional storage unit or with the hybridization of the plant

3.18.4.3 Off-Heat Usage

In all thermodynamic cycles, it is beneficial to use as much of the heat produced as possible, since this increases the efficiency of the cycle as a whole This is especially important in solar thermal power plants, since they are usually built in areas with little access to cooling water By reducing the temperature of the waste steam as much as possible, the condensation power required for the recycling of the process steam can be reduced

The sources of waste heat in a CSP plant are either steam or heated air, depending on the type of plant and individual thermodynamic cycles

Apart from the previously discussed cogeneration applications of desalination and cooling, waste heat can also be profitably used for other applications Possibly the simplest and most obvious use of this energy is the same process carried out in every thermal power plant, that is, the preheating of the feedwater for the power production cycle This is one of the highest temperature applications of waste heat and is carried out by a heat exchanger between the steam exiting the steam turbine and the feedwater entering the boiler

The applications suitable for the waste heat are determined by its source temperature Low-temperature heat (<100 °C) is suitable for hot water and space heating in residential, commercial, and institutional buildings, schools, hotels, and swimming pools as well as for crop drying

CSP plants often suffer the disadvantage of variable solar irradiance, which results in unstable steam or hot air parameters This could prove to be a severe disadvantage for applications such as crop drying, since these require very precisely controlled air temperatures However, the advantage of these lower temperature applications is that even with fluctuating power process parameters, low-temperature applications (of up to approximately 100 °C) are practically always sustainable

Higher temperature waste heat (>100 °C) can be used for industrial processes, for example, in the paper and food industries and sterilization

In the best-case scenario, solar tower power plants will be run on a combined cycle similar to many modern fossil-fueled power plants, with a GT topping and a steam turbine bottoming cycle Only then will the true potential of the technology be harnessed In this case, the waste heat of the GT will be the energy supply for the steam production for the steam turbine, and waste heat from both heated air and steam will be readily available

A further possibility for central receiver power plants is that waste heat may be obtained from the peripheral areas of the receiver, where air can be heated to 200 or 300 °C, containing enough thermal energy to drive waste heat processes but not enough to produce power in a steam turbine or GT