Hybrid PV/T solar systems for domestic hot water and electricity production

Transcription

1 Energy Conversion and Management 47 (26) Hybrid PV/T solar systems for domestic hot water and electricity production S.A. Kalogirou a, *, Y. Tripanagnostopoulos b a Higher Technical Institute, P.O. Box 2423, Nicosia 2152, Cyprus b Physics Department, University of Patras, Patra 265, Greece Received 2 January 25; received in revised form 26 July 25; accepted 3 January 26 Available online 22 March 26 Abstract Hybrid photovoltaic/thermal (PV/T) solar systems can simultaneously provide electricity and heat, achieving a higher conversion rate of the absorbed solar radiation than standard PV modules. When properly designed, PV/T systems can extract heat from PV modules, heating water or air to reduce the operating temperature of the PV modules and keep the electrical efficiency at a sufficient level. In this paper, we present TRNSYS simulation results for hybrid PV/T solar systems for domestic hot water applications both passive (thermosyphonic) and active. Prototype models made from polycrystalline silicon (pc-si) and amorphous silicon (a-si) PV module types combined with water heat extraction units were tested with respect to their electrical and thermal efficiencies, and their performance characteristics were evaluated. The TRNSYS simulation results are based on these PV/T systems and were performed for three locations at different latitudes, Nicosia (35 ), Athens (38 ) and Madison (43 ). In this study, we considered a domestic thermosyphonic system and a larger active system suitable for a block of flats or for small office buildings. The results show that a considerable amount of thermal and electrical energy is produced by the PV/T systems, and the economic viability of the systems is improved. Thus, the PVs have better chances of success especially when both electricity and hot water is required as in domestic applications. Ó 26 Elsevier Ltd. All rights reserved. Keywords: Solar energy; Photovoltaics; Thermal collectors; Hybrid photovoltaic/thermal system; Water heating 1. Introduction The temperature of PV modules is increased by the absorbed solar radiation that is not converted into electricity, causing a decrease in their efficiency. For monocrystalline (c-si) and polycrystalline (pc-si) silicon solar cells, the efficiency decreases by about.45% for every degree rise in temperature. For amorphous silicon (a-si) cells, the effect is less, with a decrease of about.25% per degree rise in temperature depending on the module design. This undesirable effect can be partially avoided by a proper heat extraction with a fluid circulation. In * Corresponding author. Tel.: ; fax: address: skalogir@spidernet.com.cy (S.A. Kalogirou) /$ - see front matter Ó 26 Elsevier Ltd. All rights reserved. doi:1.116/j.enconman

2 S.A. Kalogirou, Y. Tripanagnostopoulos / Energy Conversion and Management 47 (26) Nomenclature A a collector area (m 2 ) a-si amorphous silicon C investment cost ( ) C p specific heat (kj/kg C) c-si monocrystalline silicon C aux cost of auxiliary energy ( ) C FA cost rate of auxiliary energy ( /kj) C FL cost rate of conventional fuel ( /kj) C load cost of fuel to cover load ( ) d market discount rate (%) DT temperature difference ( C) FYFS first year fuel savings ( ) G total global solar radiation (W/m 2 ) i interest rate (%) I m current of PV module operating at maximum power (A) LCS life cycle savings ( ) _m mass flow rate (kg/s) N number of years g el electrical efficiency g th thermal efficiency p-si polycrystalline silicon PV photovoltaic PV/T hybrid photovoltaic/thermal collector PW present worth ( ) Q u useful energy extracted (kj) Q aux auxiliary energy (kj) Q load energy required to cover load (kj) T a ambient temperature ( C) T i inlet temperature to collector ( C) T o outlet temperature from collector ( C) T PV PV module temperature ( C) T PV(eff) effective PV module temperature ( C) T PV/T operating temperature of PV/T module ( C) voltage of PV module operating at maximum power (V) V m hybrid photovoltaic/thermal (PV/T) solar systems, the reduction of PV module temperature can be combined with useful fluid heating. Therefore, hybrid PV/T systems can simultaneously provide electrical and thermal energy, achieving a higher energy conversion rate of the absorbed solar radiation. These systems consist of PV modules coupled to heat extraction devices in which air or water of lower temperature than that of the PV modules is heated, while at the same time, the PV module temperature is reduced. In PV/T system applications, the production of electricity is the main priority, and therefore, it is necessary to operate the PV modules at low temperature in order to keep the PV cell electrical efficiency at a sufficient level. Natural or forced air circulation are simple and low cost methods to remove heat from PV modules, but they are less effective if the ambient air temperature is over 2 C. To overcome this effect, the heat can be extracted by circulating water through a heat exchanger that is mounted at the rear surface of the PV module. PV/T systems provide a higher energy output than standard PV modules and could be cost effective if the additional cost of the thermal unit is low. Water type PV/T systems can be practical devices for water heating (mainly domestic hot water), but they are not improved enough yet for commercial applications. Therefore, the objective of this work is to evaluate

3 337 S.A. Kalogirou, Y. Tripanagnostopoulos / Energy Conversion and Management 47 (26) the performance and financial improvement of PV/T systems compared to standard PV systems, which could prove beneficial to the greater diffusion of PV units. Also, for countries with good penetration of solar water heaters (Cyprus = 93% and Greece = 25%), where it is a habit to produce hot water with solar energy, it would be difficult to convince potential customers to install a PV system, whereas a hybrid system producing both electricity and hot water has better chances of success. The main concepts of hybrid PV/T systems have been presented in several publications from 1978 [1 5]. Other interesting applications are a low cost PV/T system with transparent type a-si cells [6] and building integrated PV/T systems [7]. Following these initial studies, the design aspects of a water type PV/T system [8] and a detailed analysis of liquid type PV/T systems [9] were presented. More recently, results from integrated PV/T systems with hot water storage [1,11] and PV/T collectors with polymer absorber [12] were given. Several other models of water cooled PV/T systems, like models for PV/T prototypes with water heat extraction modes [13,14], modeling results [15] and a study on domestic PV/T systems [16] have been presented. Finally, PV/T thermosyphonic solar water heaters were studied regarding their performance [17,18], and a commercial thermosyphonic system [19] was introduced in the market. Most of the above works give experimental and modeling results regarding the performance of PV/T systems with forced or natural flow of the heat removal fluid, but only few of them [16,19] include information on cost and energy aspects for practical applications. The work of the authors in this area includes the design and performance improvements of hybrid PV/T systems with water or air as heat removal fluid [2]. The investigated models include a number of modifications that contribute to the increase of thermal efficiency, to the decrease of PV module temperature and to the improvement of the total energy output of the PV/T system. Design concepts, prototype construction and test results for water and air cooled PV/T systems were presented for PV/T systems with and without an additional glass cover [2]. The dual type PV/T system, operating either with water or air heat extraction, extends their practical use [21], and a life cycle analysis for water cooled PV/T systems compared with standard PV modules demonstrated the environmental impact of these systems [22]. In addition, economic aspects and performance results for water cooled PV/T systems that could be applied in houses, multi-flat residential buildings, hotels, etc., show the advantages of applying PV/T systems [23]. The work of the authors includes also extensive studies of solar energy applications by using advanced simulation tools. A part of this work refers to hybrid PV/T water heaters and is based on the use of the TRNSYS simulation program where a hybrid PV/T system was modeled and simulated for the environmental conditions of Nicosia, Cyprus [24]. The system can satisfy part of the thermal and electrical needs of a family of four persons. From the results presented, it was shown that the mean annual electrical efficiency of the PV was increased considerably, and the system can satisfy 5% of the thermal needs of the family. In this paper, we present basic design considerations for hybrid PV/T systems that were investigated as prototypes that can be applied in houses, multi-flat residential buildings, hotels, etc., aiming to provide both electricity and hot water. The electrical and thermal efficiency of designed, constructed and tested prototype PV/T panels of polycrystalline silicon (pc-si) and amorphous silicon (a-si) PV modules are presented and TRNSYS simulation results are given for Nicosia, Athens and Madison for a small thermosyphon unit (4 m 2 ) suitable for a single house and an active system for large scale hot water and electricity production (4 m 2 ). The objective is to prove the potential benefits of PV/T systems compared to typical PV modules. This work provides energy and cost results regarding system application and could be considered useful to estimate the cost effectiveness of these new solar energy systems in practice. 2. Experimental PV/T systems A PV/T system consists of a thermal unit for extraction of the heat by water, which circulates through pipes in contact with a flat sheet placed in thermal contact with the rear surface of the PV module (see details in Section 3). Practical considerations in PV/T system design include evaluation of the thermal and electrical efficiency improvement with respect to system cost. It should be noted that the cost of the thermal unit remains the same irrespective of whether the PV module is made from c-si, pc-si or a-si cells, but the ratio of the additional cost of the thermal unit per PV module cost is almost double in the case where a-si modules are used rather than the c-si or pc-si PV ones. In addition, a-si PV modules present lower electrical efficiency, although the total energy output (electrical plus thermal) is almost equal to that of c-si or pc-si PV modules.

4 S.A. Kalogirou, Y. Tripanagnostopoulos / Energy Conversion and Management 47 (26) Hybrid PV/T systems consisting of PV modules without thermal protection of their illuminated surface from the ambient have high top thermal losses, and therefore, the operating temperature is not high. To increase the system operating temperature, an additional transparent cover is necessary (like the glazing of typical solar thermal collectors), but this has the result of decreasing the PV module electrical output because of the additional absorption and reflection of solar radiation. In addition, these systems use thermal insulation to avoid thermal losses from the non-illuminated system surfaces. PV/T systems can be installed on a horizontal or inclined roof or on the façade of a building. The horizontal and inclined roof installations are of more interest for low latitude countries, while the building façade installation is more effective for medium and high latitude applications because of the low sun altitude angles. Smaller size PV and PV/T systems using a PV aperture surface area of about 3 5 m 2 and a water storage tank of 15 3 l can be installed in one family houses. Larger size systems of about 3 5 m 2 and 1 3 l water storage are more suitable for multi-flat residential buildings, hotels, hospitals, industries, etc. In the case of small size PV/T systems, a thermosyphonic operation (without pump for circulation of the water) can also be used, aiming to replace the wellknown flat plate thermosyphonic solar water heaters. Two types of PV/T experimental models were constructed and tested, one with pc-si and another with a-si PV modules. The studied PV/T systems consist of PV modules in combination with water heat extraction units made from copper sheet and pipes and thermally protected with 5 cm of polyurethane thermal insulation. All models had glazing covers of 4 mm thickness to achieve satisfactory thermal output. The experimental models of the hybrid PV/T water systems were constructed and tested outdoors for determination of the steady state thermal efficiency g th and the electrical efficiency g el [2]. The thermal efficiency of the experimental PV/T models is determined as a function of the global solar radiation (G), the input fluid temperature (T in ) and the ambient temperature (T a ). The electrical efficiency of the PV/T systems is determined for all the PV module types as a function of the operating temperature T PV/T. During the testing for determination of the system thermal efficiency, the PV modules were connected with a load to simulate real system operation and to avoid PV module overheating by the solar radiation that is converted into heat instead of electricity. The steady state efficiency is calculated by: g th ¼ _mc p ðt o T i Þ=GA a ; ð1þ where _m is the fluid mass flow rate, C p the fluid specific heat, T i and T o the input and output fluid temperatures, respectively, and A a the aperture area of the PV/T model. The thermal efficiency g th of PV/T systems is calculated as a function of the ratio DT/G where DT = T i T a, with T a being the ambient temperature. The electrical efficiency g el depends mainly on the incoming solar radiation and the PV module temperature (T PV ) and is calculated by: g el ¼ I m V m =GA a ð2þ where I m and V m are the current and the voltage of the PV module operating at maximum power. The electrical efficiency of the PV cells depends on the incoming solar radiation and their operating temperature. The formula that can be used for calculation of the PV module temperature is a function of the ambient temperature T a and the incoming solar radiation G and is given by Lasnier and Ang [25] as: T PV ¼ 3 þ :175ðG 3Þþ1:14ðT a 25Þ ð3þ This relation is used for standard pc-si PV modules. In PV/T systems, the PV temperature depends also on the system operating conditions, such as the heat extraction fluid mean temperature. In PV/T systems, the PV electrical efficiency g el can be a function of the parameter (T PV ) eff, which corresponds to the PV temperature for the operating conditions of the PV/T systems. Thus, for the studied PV/T systems, we used the effective value (T PV ) eff calculated by the formula [23]: ðt PV Þ eff ¼ T PV þðt PV=T T a Þ ð4þ The operating temperature T PV/T of the PV/T system pertains to the PV module and to the thermal unit temperatures and can be determined approximately by the mean fluid temperature.

5 3372 S.A. Kalogirou, Y. Tripanagnostopoulos / Energy Conversion and Management 47 (26) For the a-si PV modules, their lower electrical efficiency results in slightly higher PV module temperature as compared to pc-si PV modules. For this purpose, the following formula, which was validated by experiments, was used: T PV ¼ 3 þ :175ðG 15Þþ1:14ðT a 25Þ ð5þ The thermal and electrical efficiencies of all the studied models are the following: pc Si : g th ¼ :71 9:4ðDT =GÞ g el ¼ :1457 :94ðT PV Þ eff ð6þ a Si : g th ¼ :75 8:83ðDT =GÞ g el ¼ :485 :11ðT PV Þ eff ð7þ For the pc-si type PV/T systems, the (T PV ) eff is calculated from Eqs. (3) and (4). For the calculation of (T PV ) eff of a-si type PV/T systems, Eqs. (4) and (5) are used. It should be noted that inverters, regulators and other auxiliary equipment that constitute the Balance of System (BoS) result in reduction of the final energy output of all systems by about 15% [22] due to electrical and thermal losses. 3. Systems considered In this paper, two types of PV/T systems are considered as follows: (a) Small size PV/T solar water heating system of thermosyphonic type. (b) Large size system with PV/T modules in parallel rows placed on a horizontal building roof with the water storage tank located inside the building and a pump for the water circulation. Thermosyphon systems heat potable water or a heat transfer fluid. These systems need no pumps and controls to transfer the water heated by solar energy, as they use natural convection to transport it from the collector to storage. The water in the collector expands, becoming less dense as the sun heats it, and rises through the collector into the top of the storage tank [26]. There, it is replaced by the cooler water that has sunk to the bottom of the tank, from which it flows down the collector, and circulation continues as long as there is sunshine. In thermosyphon systems, the collector is connected with a water storage tank which is always at a higher position so as to avoid reverse operation during the night. Active systems on the other hand use a pump to circulate the water from the collector to storage. A typical active system is shown in Fig. 1. The pump is operated by means of a differential thermostat. The storage tank can be located at any place, like behind the collectors, indoors in a plant room or any other suitable location, and thus, there is an overall improvement in the aesthetics of the system. To avoid water freezing in the tubes of the collector, a heat exchanger is used in the storage tank, and the heat removal fluid is water with antifreeze liquid. The specifications of the small solar system are shown in Table 1, whereas the specifications of the large system are shown in Table 2. Relief valve Mixing device Hot water supply Collector array Differential thermostat Storage tank Auxiliary heater Burner Solar pump Make-up water Fig. 1. Active system schematic.

6 S.A. Kalogirou, Y. Tripanagnostopoulos / Energy Conversion and Management 47 (26) Table 1 Specification of the small solar system considered Parameter Specification Collector area 4m 2 Collector slope Latitude + 5 Storage capacity 16 l Auxiliary capacity 3kW Hot water demand 12 l (4 persons) Table 2 Specification of the large solar system considered Parameter Specification Collector area 4 m 2 Collector slope Latitude + 5 Storage capacity 15 l Auxiliary capacity 1 kw Hot water demand 12 l (4 persons) The usual type of collector employed in both thermosyphon and active units is the flat plate. A typical flat plate solar collector is shown in the detail of Fig. 2. The underside of the absorber plate and the side of the casing are well insulated to reduce conduction losses. For the present application, PV cells are installed on top of the absorber, as shown in Fig. 2, so the absorber is the PV panel and the copper absorber acts as a heat removal medium. The transparent cover is used to reduce convection losses from the absorber plate through the restraint of the stagnant air layer between the absorber plate and the glass. It also reduces radiation losses from the collector. Although the transparent cover reduces the amount of electrical energy produced by the PV panel, it is retained for the present applications so as to increase the thermal performance of the collectors. Natural or forced air circulation is a simple and low cost method to remove heat from PV modules, but it is less effective at low latitudes where ambient air temperatures are over 2 C for many months of the year. Regarding heat extraction, the water circulates through pipes in contact with a flat sheet (heat exchanger) placed in thermal contact with the PV module rear surface as shown in Fig. 2. The additional thermal protection increases the thermal efficiency of the system, but the lower thermal losses keep the PV temperature at a higher level, therefore operating with reduced electrical efficiency. To increase the system operating temperature, an additional glazing is used, but this results in a decrease of the PV module electrical output because an amount of solar radiation is absorbed by the glazing and another is reflected away, depending on the angle of incidence. The characteristics of the solar collector considered in this study are shown in Table 3. In PV/T systems, the collector needs to be connected electrically to the mains (for grid connected systems) and hydraulically to a hot water storage tank. Two types of PV cells have been considered in this work polycrystalline silicon (pc-si) and amorphous silicon (a-si). Fig. 2. Hybrid and conventional flat plate collector details.

7 3374 S.A. Kalogirou, Y. Tripanagnostopoulos / Energy Conversion and Management 47 (26) Table 3 Characteristics of the solar collector considered Parameter Riser pipe diameter Header pipe diameter Absorber plate thickness Insulation material and thickness Fixing of risers on the absorber plate Glazing Characteristics 15 mm 28 mm.5 mm Fiber wool, 4 mm Welded Low-iron glass 4. System model The system is modeled with the well-known TRNSYS program [27]. The program consists of many subroutines that model subsystem components. Once all the components of the system have been identified and a mathematical description of each component is available, it is necessary to construct an information flow diagram for the system. The purpose of the information flow diagram is to facilitate identification of the components and the flow of information between them. From the flow diagram, a deck file has to be constructed containing information on all system components, weather data file and the output format. In a previous work in which an active PV/T system was modeled, it was found that the optimum water flow rate value was 4.9 l/h m 2 [24]. This low value of flow rate suggests that the system could be used in a thermosyphon mode. For simulation of the thermosyphon system, the TRNSYS Type 45 model is used. The thermal performance of the system is analyzed by dividing the thermosyphon loop into a number of segments normal to the flow direction and applying Bernoulli s equation for incompressible flow to each segment. The flow rate is obtained by numerical solution of the resulting set of equations. Flow in the loop is assumed to be steady state. For the active system, the units are modeled by combining a number of components as shown in Fig. 3, which shows the flow diagram of the large hot water application. In all the deck files required to run the TRNSYS models for the various applications, all equations shown in Section 2 are incorporated, and whenever possible, outputs from ready made modules were used directly. For example, the mass flow rate (for the thermosyphon system) and collector inlet and outlet temperatures are obtained in this way and used in the appropriate equations. All systems are simulated on an annual basis at three different locations at different latitudes, Nicosia, Cyprus (35 ); Athens, Greece (38 ) and Madison, Wisconsin (43 ). The first two locations represent locations TMY file TYPE 9 DATA READER Ambient temperature LEGEND: Information flow Control signal flow Raw data input TYPE 6 AUXIL IARY HEATER EQUATIONS (for the PV system) TYPE 16 SOLAR RADIATION PROCESSOR TYPE 1 SOLAR COLLECTOR Solar radiation TYPE 31 PIPING TYPE 2 COLLECTOR CONTROL TYPE 3 COLLECTOR PUMP TYPE 14 Mains water temperature TYPE 4 TYPE 11 TEE PIECE TYPE 11 DIVERTER HOT WATER STORAGE T out TYPE 31 PIPING TYPE 15 RELIEF VALVE EQUATIONS (for the TPV-T) T in TYPE 3 LOAD PUMP TYPE 14 Hot water load Fig. 3. Information flow diagram for the large hot water system application.

8 S.A. Kalogirou, Y. Tripanagnostopoulos / Energy Conversion and Management 47 (26) Hot water consumption (l) Hours Fig. 4. Hot water daily consumption profile. with hot summer weather and mild winters, whereas the latter represents a location with mild summer and severe winter and was considered to find the difference in system performance for comparison purposes. For each of these three locations, a typical meteorological year (TMY) file, which is required in the simulations, is available. With respect to the water consumption and although the hot water demand is subject to a high degree of variation from day to day and from consumer to consumer, it is impractical to use anything but a repetitive load profile. This is not quite correct during the summer period where the consumption pattern is somewhat higher. However, during this period, the temperature requirement for hot water is not as high as during the winter. Consequently, the total thermal energy requirement is reasonably constant throughout the year. For the present simulation, the hot water consumption profile illustrated in Fig. 4 is used, which assumes a daily hot water consumption of 12 l at 5 C for a family of four (3 l/person). For the large hot water application, the consumption considered is 1-fold of that shown in Fig. 4 (1 families of four persons each). 5. Results The annual results obtained for the thermosyphon unit are shown in Table 4. As can be seen, the PV/T systems achieve an increase of the total energy output because hybrid systems utilise the thermal energy, whereas in a standard PV system, this is lost to the ambient. However, the electrical energy output of a hybrid system is lower than that of standard PV modules (maximum 38%) due to operation of the PV modules at higher temperatures. The reduced electrical performance is also due to the additional glazing, which increases the thermal output and the optical losses. Generally, depending on the location, this system can give kw h electrical energy, and the solar contribution varies from 29% to 72%. The solar contribution determines the percentage of the hot water load covered by solar energy. The pc-si PV modules give higher total energy output compared to a-si PV modules. However, the a-si gives more thermal useful energy and, thus, a higher solar contribution in water heating. In cold climates Table 4 Annual performance of the hybrid PV/T thermosyphon system Location Cell type Q u (MJ) Q aux (MJ) Solar fraction PV/T electrical energy (kw h) PV electrical energy (kw h) Electrical energy % difference Nicosia pc-si a-si Athens pc-si a-si Madison pc-si a-si Notes: (1) Q u = useful thermal energy. (2) Q aux = auxiliary thermal energy required to cover hot water load. (3) Electrical energy is estimated by considering an 85% efficiency for BoS.

9 3376 S.A. Kalogirou, Y. Tripanagnostopoulos / Energy Conversion and Management 47 (26) and although the overall performance of the hybrid system is reduced due to the excessive cloudiness, the comparative performance of the PV is better because of the operation at a lower environmental temperature. The annual results for the thermosyphon PV/T system are shown in Table 4 and the monthly performance in Fig. 5. As can be seen, on a monthly basis, pc-si cells produce more electrical energy (P el ) than the corresponding a-si cells. This is due to the higher efficiency of the pc-si cells. The a-si cells produce more thermal useful energy (Q u ) in all three locations considered. For Nicosia and Athens, both types of cells cover all thermal energy required for hot water production in the summer months as represented by the zero or near zero auxiliary energy required (Q aux ). For Madison, where the temperatures and available solar radiation are lower, a substantial amount of thermal energy is covered in the summer, but some thermal auxiliary is still required. All systems represent a substantial thermal energy collection and a good electrical performance throughout the year. It should be noted that although the model used for the thermosyphon unit cannot work with a heat exchanger, in practice, a mantle-type tank and a water with antifreeze liquid needs to be used in locations where freezing is a possibility, like in Madison, with a small reduction (about 15%) in the thermal performance of the unit. The annual results for the large active system are shown in Table 5 and the monthly performance in Fig. 6. Much higher values of electrical energy are obtained for the large systems, in the order of 1-fold that of the smaller systems. The solar contribution of the PV/T systems varies, depending on the location, between 6% and 87%. Also, the percentage difference between the electrical energy produced from the standard and the hybrid units is smaller, in the order of 38%. This has been estimated by considering also the energy required Q u-pc Q aux-pc P el-pc Q u-a Q aux-a P el-a Energy (MJ) Energy (MJ) Energy (MJ) Nicosia Months Athens Months Madison Months Fig. 5. Monthly performance of small (thermosyphon) hybrid PV/T system.

10 S.A. Kalogirou, Y. Tripanagnostopoulos / Energy Conversion and Management 47 (26) Table 5 Annual performance of the large hybrid PV/T active system Location Cell type Q u (MJ) Q aux (MJ) f (%) PV/T electrical energy (kw h) PV electrical energy (kw h) Pump energy (kw h) Electrical energy % difference Nicosia pc-si 52, a-si 54, Athens pc-si 46,48 14, a-si 48,97 13, Madison pc-si 37,81 21, a-si 4,78 19, Notes: (1) Q u = useful thermal energy. (2) Q aux = auxiliary thermal energy required to cover hot water load. (3) f = solar fraction, denotes percentage of hot water load covered by solar. (4) Electrical energy is estimated by considering an 85% efficiency for BoS and pump energy. Q u-pc Q aux-pc P el-pc Q u-a Q aux-a P el-a Energy (MJ) Energy (MJ) Energy (MJ) Nicosia Months Athens Months Madison Months Fig. 6. Monthly performance of large hybrid PV/T active system. by the solar pump. As the other comments about the comparison between the various systems are similar to the ones made for the smaller system, they will not be repeated again. A general comment on the monthly performance of the systems indicated in Fig. 6 is that the useful thermal energy (Q u ) of the amorphous cells is slightly higher than the corresponding value of the polycrystalline cells by an equal amount in each month, which is also reflected in the auxiliary thermal energy required to cover the hot water load. However, the electrical energy produced by the polycrystalline cells is much higher than that of the amorphous ones due to their higher electrical efficiency.

11 3378 S.A. Kalogirou, Y. Tripanagnostopoulos / Energy Conversion and Management 47 (26) Economic analysis The viability of all the above systems depends on the initial cost and the amount of energy, electrical or electrical plus thermal, depending on the type of system they replace. A life cycle analysis is performed in order to obtain the total cost (or life cycle cost) and the life cycle savings (LCS) of the systems. The economic scenario used in this project is that 1% of the initial cost of the solar system is paid at the beginning, i.e., there is no mortgage payment. The period of economic analysis is taken as 2 years (life of the system). Although early PV installations showed that the life of the PVs is more than 3 years, the 2 years period applied in solar thermal systems is retained. The economic analysis can be performed either within the TRNSYS environment or in a spreadsheet program. For the present work, the spreadsheet application is used. A detailed description of the method of economic analysis of solar systems using spreadsheets is given in Ref. [28]. In general, the present worth (or discounted cost) of an investment or cost C at the end of year N at a discount rate of d and interest rate of i is obtained by: Cð1 þ iþn 1 PW N ¼ ð1 þ dþ N ð8þ In the case of this project, the various costs and savings are estimated annually. From the addition of electricity and fuel savings incurred because of the use of the system and the tax savings, the maintenance and parasitic costs are subtracted, and thus, the annual solar savings of the system are estimated, which are converted into present worth values of the system. These are added to obtain the life cycle savings according to the equation: PW LCS ¼ XN N¼1 Solar Savings ð1 þ dþ N ð9þ For the thermal part of the system, the fuel savings are obtained by subtracting the annual cost of the conventional fuel used for the auxiliary energy from the fuel needs of a fuel only system. The integrated cost of the auxiliary energy use for the first year, i.e., solar back up, is given by the formula: C aux ¼ Z t C FA Q aux dt ð1þ The integrated cost of the total load for the first year, i.e., cost of conventional fuel without solar, is: C load ¼ Z t C FL Q load dt ð11þ where C FA and C FL are the cost rates for auxiliary energy and conventional fuel, respectively. In case the same fuel is used for both, C FA = C FL. The investment cost of the solar systems is estimated by considering the current costs of the various parts of the systems (PV module, heat extraction unit, inverter, pipes, pump, cables, etc.). These are tabulated in Table 6 together with explanations on how the costs of the various systems are estimated. As can be seen, the specific costs for the larger systems are slightly lower due to economy of scale. For example, for the larger system, one slightly larger inverter is used, which is slightly more expensive than the one used in the smaller systems. It should be noted that the cost of the hybrid PV/T systems includes also the costs for modification of the PV systems into the hybrid ones and all other equipment such as piping, pump, differential thermostat and insulation required to complete the system. It does not include the cost of the storage tank, which is present in an installation irrespective of whether this is solar or not. Thus, in the economic analysis, a comparison is made of the extra equipment required by the solar system against the money saved because of the amount of electricity and fuel replaced by solar energy. As subsidization schemes for PV systems vary from country to country and as the economic analysis is performed mainly in order to compare the standard and hybrid systems, no subsidies are considered in the present analysis.

12 S.A. Kalogirou, Y. Tripanagnostopoulos / Energy Conversion and Management 47 (26) Table 6 Specific costs of the systems considered System description PV panel Cost ( ) Explanation of costs Thermosyphon system (4 m 2 ) 9 12 for complete system 3 for storage tank Thermal system (per m 2 ) 25 Includes collectors, piping, insulation and supports Small PV system (4 m 2 ) pc-si 32 Current market price which includes cost for BoS Small PV system (4 m 2 ) a-si 2 Current market price which includes cost for BoS Large PV system (4 m 2 ) pc-si 28, Current market price which includes cost for BoS Large PV system (4 m 2 ) a-si 16, Current market price which includes cost for BoS Thermosyphon PV/T system (4 m 2 ) pc-si for PV + 9 for solar thermal system Thermosyphon PV/T system (4 m 2 ) a-si for PV + 9 for solar thermal system Large PV/T system (4 m 2 ) pc-si 38, 4 7 for PV for solar thermal system Large PV/T system (4 m 2 ) a-si 26, 4 4 for PV for solar thermal system Note: Storage tank is assumed to be present in a conventional hot water system, thus it is not considered as cost in the present analysis, i.e., only the cost of extra equipment is considered. For the operating cost, maintenance and parasitic costs are considered. The former are estimated to be 1% of the initial investment and are assumed to increase at a rate of 1% per year of the system operation. The latter accounts for the energy required (electricity) to drive the solar pump. Thus, the total annual cost is given by the addition of the system and operation costs. The cost of electricity is considered as.1 /kw h, and the cost of Diesel is.62 /l. The market discount rate and the general inflation rate considered in this study are equal to 6.5% and 5.2%, respectively. Finally, the inflation rate for the fuel used is equal to 1% to reflect the latest dramatic increases in oil price. The results of the economic analysis are shown in Table 7 for the small system and Table 8 for the large system. What is of interest here is mainly the comparison between the savings in electricity and thermal energy and the LCS of the various types of systems. As can be seen in all the cases of the PV systems, the LCS obtained are negative, meaning that the payback time is greater than 2 years, which is considered as the life of the systems. It should be noted that no subsidies are considered in this study and the negative amounts of money represent the money that the owner will loose by installing the PV system instead of buying the electricity from the mains. These negative figures show the need for subsidies in order to convince people to install such systems. It should be noted that the LCS strongly depends on the first year fuel savings (FYFS) and the solar system cost. The electrical FYFS (FYFS-e) depend on the electrical energy produced by the PVs. In hybrid systems, the thermal FYFS (FYFS-t) depend on the thermal load in each case and the auxiliary energy required. Better figures are obtained in the case of hybrid systems, as the LCS are smaller negative values, and in some cases, positive values are obtained. All cases that give positive LCS refer to the use of a-si cells, and these are the applications for Nicosia. Generally, for locations with higher available solar radiation, like Nicosia and Athens, the economics give better figures. Also, although amorphous silicon panels are much less efficient than the polycrystalline ones, they give better figures Table 7 Results of the economic analysis of the small system System type (Location) Cell type Standard PV cells Hybrid PV/T cells FYFS-e electricity LCS FYFS-e electricity FYFS-t fuel LCS FYFS-t electricity Thermosyphon (Nicosia) pc-si a-si Thermosyphon (Athens) pc-si a-si Thermosyphon (Madison) pc-si a-si Notes: (1) All values in Euros. (2) FYFS = first year fuel savings. (3) LCS = life cycle savings. (4) FYFS-e = electricity replaced by solar during first year. (5) FYFS-t = thermal energy replaced by solar during first year when backup is fuel (Diesel) or electricity. LCS

13 338 S.A. Kalogirou, Y. Tripanagnostopoulos / Energy Conversion and Management 47 (26) Table 8 Results of the economic analysis of the large system Location Cell type Standard PV cells Hybrid PV/T cells FYFS-e electricity LCS FYFS-e electricity FYFS-t fuel LCS Nicosia pc-si a-si Athens pc-si a-si Madison pc-si a-si Notes: (1) All values in Euros. (2) FYFS = first year fuel savings. (3) LCS = life cycle savings. (4) FYFS-e = electricity replaced by solar during first year. (5) FYFS-t = thermal energy replaced by solar in first year when backup is fuel (Diesel). (6) All systems are active. due to their lower initial cost, i.e., they have better cost/benefit ratios, as has also been observed by other authors [16]. For the case of the small system, both Diesel and electricity are used as a backup auxiliary for the thermal energy. This is to account for houses that produce hot water with Diesel through the central heating system and those that use electric immersion heaters. As the unit cost of electricity is higher that that of fuel, in the case where an electric immersion heater is used, the solar energy replaces a more expensive fuel, and thus, a higher LCS is obtained. The modeled energy output results of the considered PV/T systems are generally in agreement with the results that are presented by other authors. In the literature on PV/T solar systems however, no monthly or annual results for these systems are given. A recent publication considers 4 m 2 c-si and a-si PV/T systems with glazing and 3 l water storage tank. The systems are studied with TRNSYS [16], but only a few annual results are given, showing that the effective application of the two cell types depends on the electrical to thermal energy value ratio. Regarding the benefits of commercial PV/T solar water heaters [19], it is estimated that, in the case of cost reduction by mass production and considering that the produced heat replaces electricity, the system becomes cost effective for practical applications. Taking into account these aspects, an example is given by considering a subsidy of 55% on the initial cost of the PV system, which is presently applied in the case of Cyprus as an incentive to promote PV systems installation. The LCS for the polycrystalline silicon panels increase to 723 for the standard PV system, to 852 for the hybrid PV/T thermosyphon system with Diesel backup of the thermal energy and to 1733 for the hybrid PV/T thermosyphon system with electricity backup. It should be noted that in the case of the hybrid systems, the subsidy is considered only on the PV part of the system. In Cyprus, another incentive is given, namely that for grid connected systems, the electrical energy is bought by the Electricity Authority at double the normal selling rate. If this is considered, then the LCS increases to 2913 for the standard polycrystalline silicon panels, to 2233 for the hybrid PV/T thermosyphon system with Diesel backup of the thermal energy and to PV PV/T Payback time (years) Nic-p Nic-a Ath-p Ath-a Mad-p Mad-a Nic-p Nic-a Ath-p Ath-a Mad-p Mad-a Fig. 7. Payback times of plain PV and thermosyphon PV/T systems with polycrystalline (p) and amorphous (a) silicon cells for the three locations (electricity backup).

14 S.A. Kalogirou, Y. Tripanagnostopoulos / Energy Conversion and Management 47 (26) PV PV/T Payback time (years) Nic-p Nic-a Ath-p Ath-a Mad-p Mad-a Nic-p Nic-a Ath-p Ath-a Mad-p Mad-a Fig. 8. Payback times of plain PV and active PV/T systems with polycrystalline (p) and amorphous (a) silicon cells for the three locations (Diesel backup). for the hybrid PV/T thermosyphon system with electricity backup. Therefore, it can be concluded that subsidies are a must for the introduction of both standard PV and hybrid PV/T systems. Finally, in Figs. 7 and 8, the payback times in years of the systems studied are shown. In these figures, the advantage of the hybrid PV/T systems is shown, as much shorter times are indicated. Also, by comparing the polycrystalline and the amorphous silicon cells, the latter are slightly better for the hybrid type as they have better cost/benefit ratios, as outlined above. 7. Conclusion Hybrid photovoltaic/thermal (PV/T) systems consisting of pc-si and a-si PV modules combined with water heat extraction units, which were constructed and tested at the University of Patras are modeled and simulated with the TRNSYS program. The work includes the study of two PV/T systems, a small scale unit of 4 m 2 aperture area and 16 l water storage tank and a large scale system of 4 m 2 aperture area and 15 l storage tank. The results show that the electrical production of the system employing polycrystalline solar cells is more than that employing the amorphous ones, but the solar thermal contribution is slightly lower. A non-hybrid PV system produces about 38% more electrical energy, but the present system covers also, depending on the location, a large percentage of the hot water needs of the buildings considered. The derived TRNSYS results give an account of the energy and cost benefits of the studied PV/T systems with thermosyphon and forced water flow. As a general conclusion, it can be said that as the overall energy production of the units is increased, the hybrid units have better chances of success. This is also strengthened by the improvement of the economic viability of the systems, especially in applications where low temperature water, like hot water production for domestic use, is also required. Additionally, the economics of the systems considered show that for locations with higher available solar radiation, like Nicosia and Athens, the economics give better figures. Also, although amorphous silicon panels are much less efficient than the polycrystalline ones, they give better figures due to their lower initial cost, i.e., they have better cost/benefit ratios. Considering the case of Cyprus, a considerable increase in LCS can be obtained when subsidies are considered, indicating the need of state subsides in order to promote the installation of these systems. The same order of subsidies needs to be given for the hybrid systems as well. References [1] Kern EC Jr., Russel MC. Combined photovoltaic and thermal hybrid collector systems. In: Proceedings of the 13th IEEE photovoltaic specialists, Washington DC, USA, p [2] Hendrie SD. Evaluation of combined photovoltaic/thermal collectors. In: Proceedings of the international conference ISES, vol. 3, Atlanta, Georgia, USA, May 28 June 1, p [3] Florschuetz LW. Extension of the Hottel Whillier model to the analysis of combined photovoltaic/thermal flat plate collectors. Solar Energy 1979;22:361 6.

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