Acta Polytechnica https://doi.org/10.14311/AP.2024.64.0042 Acta Polytechnica 64(1):42–51, 2024 © 2024 The Author(s). Licensed under a CC-BY 4.0 licence Published by the Czech Technical University in Prague OUTDOOR PERFORMANCE INVESTIGATION OF A THERMOELECTRIC COOLER-INTEGRATED SOLAR AIR HEATING COLLECTOR Josué R. Segnon∗, Howard O. Njoku University of Nigeria, Sustainable Energy Engineering Research Group, Department of Mechanical Engineering, 410001 Nsukka, Nigeria ∗ corresponding author: josue.segnon.pg90595@unn.edu.ng Abstract. Solar air heating systems are continuously being improved by combining them with other energy conversion technologies. In this paper, outdoor tests were carried out on a thermoelectric heat-pumping solar air heater (TE-SAH), with four (04) TECs (TEC1–12706) attached to the backside of the absorber plate, and powered by a 40 Wp mono-Si PV module to pump heat from the absorber plate into the heated air. The thermal energy production, energy efficiency, heat loss coefficients, and heat removal factor were evaluated and compared with a reference system without TECs. The heat collection and energy efficiency of the solar air heater were improved by 7.14 % and 66.71 %, respectively, with the integration of TECs. Heat losses also decreased by 0.46 MJ. Furthermore, the estimated heat removal factor for the TE-SAH was 0.55, higher than 0.49 obtained for the reference SAH. These results showed that PV-TE heat-pumping is a viable means of improving the thermal performance of solar air heaters. Keywords: Solar air heating, thermoelectric cooler, heat pumping, heat collection, energy efficiency. 1. Introduction Renewable energy is at the forefront of the ongoing transition towards a cleaner and sustainable energy supply. Among the renewable energy sources, solar en- ergy is the most preferred option for use globally due to the relatively low maintenance costs of associated technologies [1]. Solar radiation can also be converted into useful heat by use of solar heating systems for ap- plication in cooling, heating, crop drying, ventilation, cooking, etc. [2, 3]. In solar heating systems, water, air, nanofluids, or any combination can be used as the working fluid. In solar air heating systems, the solar air heater (SAH) captures the solar radiation on a blackened surface (absorber plate), thus heating the plate, and transferring this heat to the working air that absorbs useful heat as it flows past the plate. Systems that combine solar technologies with other energy conversion technologies have considerably in- creased energy outputs compared to the standalone solar energy conversion technologies [4, 5]. For in- stance, photovoltaic-thermal (PVT) systems produce both thermal and electrical energies from the same absorbing surface and achieve higher energy conver- sion efficiencies than standalone PV or solar thermal systems [6, 7]. Also, thermoelectric modules (TEMs), when coupled with PV modules, produce additional electrical power, hence achieving a higher output than both the standalone PV and TEMs. Thermoelectric generators (TEGs) are semiconduc- tor devices that directly convert a temperature dif- ference into electricity, based on the Seebeck effect. This concept has been presented as a means of im- Figure 1. Schematic of an integrated solar PV-TEG system [8]. proving the electrical energy output by attaching the TEGs at the back of PV modules, thus converting the temperature difference between the PV module and the adjacent air into additional electricity, as shown in Figure 1 [5]. Thus, with the same PV absorbing surface, this combination could improve the system power output by up to 15 % [8–11]. The same principle is applied in PVTs, increasing the overall efficiency of the module [12–14]. However, this combination only converts the temperature difference into electric- ity without any focus on the thermal output of the system. The heat is not removed from beneath the ab- sorbing surface, negatively affecting the performance of the system. Based on the Peltier effect, TE modules (TE cool- 42 https://doi.org/10.14311/AP.2024.64.0042 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 64 no. 1/2024 Solar air heating collector outdoor performance ers in this case), can perform either heating or cool- ing when powered with electricity produced by e.g. PV modules (Figure 2). This offers the features of compactness, easy mobility, and environmental- friendliness in surface cooling [15–17]. Luo et al. [18] numerically compared the performance of a building- integrated PV wall (BIPV) and a building-integrated PV-TE wall (BIPVTE) to that of a massive wall. Compared to the massive wall, the energy saving ra- tio of the BIPVTE was reported to be up to 172 % higher, while that for the BIPV was just 40 % higher. Wang et al. [19] also reported a BIPVTE system that could achieve an annual CO2 emission reduction of 4305.4 kg/year. TE coolers have been used to pump heat from a PV module to lower its temperature by 18 °C and improve the electrical output of the system by 2–3 % [20, 21]. Thus, applying the TE cooling could also improve the performance of solar energy conversion systems. Solar-TE systems have also been widely applied in water desalination, cold storage, refrigeration, and air conditioning, etc. [22–24]. Allouhi et al. [25] reported that space heating using PV-TE integrated walls with 12 TE modules heated a 3 × 3 × 2.8 m room from 24 °C to 37.8 °C. The system achieved a maximum COP of 2.0 and an annual energy saving 64.0 % higher than the conventional electric heating system. Cai et al. [26] modelled the air cooling and water produc- tion using TE heat pumping and reported that by increasing the input power to the TE modules, the total cooling load increased but the system efficiency decreased significantly. In solar air heaters, the system performance largely depends on the effectiveness of the heat extraction from the absorber plate. The existing methods for increasing heat extraction include the use of fins, baf- fles, porous absorbers, multiple pass designs, and an increase in the airflow rate. With these methods, SAH energy efficiencies have been improved by up to 61 % [27–29]. SAHs can also be combined with PV modules to form a PVT collector which provides both electricity and heat. In addition to the heat produced by the PVT’s thermal collector, heat is removed from the PV module as well, cooling it, and thus improving both thermal and electrical energy outputs. Early PVT designs used fluid coolants, air, and later liquids, such as water and glycols, as heat exchange fluids. Other considerations have led to the use of PCMs, nano-fluids, heat pipes, micro-scale heat exchangers, and thermoelectric modules [30, 31]. The combined energy efficiency (electrical and thermal) of such hy- brid systems ranges from 31 % to 94 %, as reported by Diwania et al. [4] and Oner et al. [32]. Although TE heat pumping has been shown to im- prove the performance of heating/cooling systems, the existing literature for solar-TE cooling has focused on improving the electrical performance of PV modules, and no work, to date, extensively studied TE heat extraction from SAH’s absorber plate. In this paper, a Figure 2. Schematic of a solar-powered TE cooling system [33]. novel approach to improving the thermal performance of SAHs using TE heat pumping has been investi- gated experimentally. The solar irradiance incident on the collector is converted into both heat and elec- tricity by its absorber and a PV module, respectively. The electricity produced by the PV module is used to power TECs with their cold side attached to the absorber plate. These TECs then pump heat from the absorber plate and transfer it to the adjacent air flowing through the collector. The heat is then re- leased at the hot end and dissipated into the working air flowing through the collector. Hence, the heat collection is improved. The absorber plate is cooled by the TEC heat pumping, lowering the temperature gradient between the absorber plate and the ambient, thus reducing heat losses to the ambient. This results in a better performance of the system. The energy flow diagram of the proposed system is presented in Figure 3. The input of the system is solar radiation, and the output is the thermal energy, improved by the PV-powered TE heat pumping. Figure 4 shows a graphical comparison of a TE heat pumping (HP) solar air heater and a conventional SAH. In the con- ventional SAH, elevated absorber temperatures cause significant heat losses to the atmosphere. In the TE- HP-SAH, TECs are powered by the PV module to pump heat from the absorber plate. The heat is then dissipated into the air channel to reduce the absorber temperatures and lower heat losses to the atmosphere. Thus, the heat gain of the air is increased. 2. Materials and methods 2.1. System description The solar air heater construction consisted of a glazed wooden casing with 25 mm insulation on its sides and bottom to reduce conduction losses to the ambient. The space beneath the absorber plate served as a channel for airflow through the collector. A 40 Wp mono-Si PV module was placed adjacent to the ther- mal collector. The overview of the system is shown in Figure 5. The PV module powers four (04) TE coolers (TEC1- 12706) attached to the back of the absorber plate as shown in Figure 6. Fins, acting as heat sinks, were attached to the hot side of the TECs to enhance the heat transfer to the working fluid. The absorbing sur- face of the collector was 430 mm × 430 mm and the 43 Josué R. Segnon, Howard O. Njoku Acta Polytechnica Figure 3. Energy balance in the TE heat pumping SAH Figure 4. System description Figure 5. Overview design of the TE-heat pumping solar air heater air channel was 430 mm × 45 mm. The airflow was designed to collect heat from both the PV module and the absorber plate while flowing beneath them, whereas the space between the absorber plate and the glazing was sealed to minimize thermal re-radiation from the absorber plate to the atmosphere. Table 1 presents the materials used for constructing the sys- tem. Table 2 presents the specifications of the TE Figure 6. Cross-sectional view of the TE-heat pump- ing solar air heater modules and the PV module used. Figure 7 shows the arrangement of the TECs at the backside of the absorber plate. The cold sides of the TECs were in contact with the back surface of the absorber plate while heat sinks were attached to the hot sides of the TE module to increase the heat transfer to the working fluid (air). 2.2. Experimental methods Two models of the system were constructed and tested in real conditions in Nsukka (6.8429◦N, 7.3733◦E), Nigeria. One was a conventional SAH with no TEC attached to the back of the absorber plate. This 44 vol. 64 no. 1/2024 Solar air heating collector outdoor performance Designation Specifications Glazing A/R coated, 3 mm thick Casing Wood Absorber plate Al., 430 mm 430 mm, (Figure 7) Insulation Polystyrene, 25 mm thick TE module See Table 2 PV module See Table 2 Table 1. Materials specifications for SAH construction TE module Model TEC1-12706 Couples N 127 Dimensions [mm] 40 × 40 × 4.2 ∆T max [K] 68 Qmax[W ] 63 Operating temperature Tmax [°C] 138 °C Operating current [A] 6 A max Operating voltage [V] 14.4 V max PV module Model HU 40 Cell type Mono-Si Pmax[W] 40 ±3 % Cell efficiency [%] 17.3 Dimensions [mm] 670×430×22 Table 2. TE and PV modules specifications prototype served as the control system for evaluating the performance of the second model with TE heat pumping. In this paper, the tests on the reference SAH will be referred to as Scenario A while the tests on the TE-SAH will be referred to as Scenario B. Natural air circulation through the collectors was adopted in this study. The electrical output of the PV module was connected to the TE modules which were connected in series of two to ensure that the maximum output voltage of the PV module was less than the maximum operating voltage of a single TEC. The electrical arrangement of the TECs is presented in Figure 8 and as such, the current and voltage input to each TE module was: Iin = Iout 2 , (1) Vin = Vout 2 , (2) yielding the power input to each TEC as Pin = Iout 2 Vout 2 = Pout 4 . (3) This showed that the power input to the TECs could be improved by improving the PV energy output. This PV output could increase with better weather condi- tions or through the use of a PV module of a higher rating, supplying more power to the TECs, which would perform better and improve the performance of the system. A data logging solarimeter (TES-1333R) was used to measure the insolation incident on the absorbing Figure 7. TECs arrangement on the absorber plate. 1. Absorber plate; 2. TE Cooler; 3. Heat sinks. Figure 8. Electrical connection of the TECs surface, a digital multimeter (UNI-T UT61 C) was used to measure the output voltage and current of the PV module, and a hot wire anemometer (BENETECH GM 8903) was used to measure the velocity of the air flowing through the collector. A multichannel temperature meter (APPLENT AT4208) was used to measure the temperature at different points (inlet, outlet, absorber plate, TE ends) of the SAH. The collector was tilted at an angle of 25° in the hor- izontal direction, determined by previous works in the same location [3, 34] as the optimal angle. The setup (presented in Figure 9) was monitored from 8:00 to 16:00 GMT and the data collected were an- alyzed to investigate the performance of the system. Due to the availability of only one set of measuring instruments, only one prototype with interchangeable absorber plates (one with TEC, and the other with- out) was constructed. Thus, the data collection was done on different days. 2.3. Performance analysis The collected data were analysed to evaluate the per- formance of the systems. The heat gain and the thermal energy efficiency were of interest in this study. The thermal energy production rate is computed as: Qu = ṁc(Tout − Tin), (4) 45 Josué R. Segnon, Howard O. Njoku Acta Polytechnica Figure 9. Experimental setup ṁ = ρvAd, (5) where v is the air velocity [m s−1], and Ad is the cross- sectional area of the collector [m2]. The power output of the PV module is determined as: Pout = IoutVout. (6) The instantaneous thermal energy efficiency relates the heat output to the energy input, and is obtained by: ηth = Qu SAc , (7) where S is the irradiance [W m−2]. Alternatively, the instantaneous efficiency of the solar air heater can be expressed as [35]: ηth = FR(τeαc) − FRUL Tav − Tamb S , (8) where αc is the absorptance of the absorber plate (αc = 0.85 for black coated galvanised sheet, τe = 0.96 for A/R coated glass used as glazing [36]), FR is the heat removal factor, UL is the heat loss factor, Tav = Tout+Tin 2 is the average temperature of the collector. From Equation 8, the heat removal factor can be calculated as FR = ηth αcτe − UL Tav−Tamb S . (9) The cumulative efficiency relates the cumulative heating rate to the cumulative energy input rate to the system and is computed using Equation 10 [37] ηcum = ∫ t 0 ṁc(Tout − Tin)dt∫ t 0 (SAc)dt , (10) where Qu is the air heating rate (W), ṁ is the mass flow rate (kg/s) (Equation 5), c is the specific heat ca- pacity of air [ JK−1kg−1] , Tout is the collector output temperature (K), and Tin is the inlet temperature [K]. ρ is the density of air [ kg m−3] . As in the TE-SAH, the power output of the PV module is completely channeled into the TECs, as presented in Figure 3, only thermal energy output is obtained from this sys- tem. The overall losses of by the thermal collector Qloss could be evaluated for estimation of the overall heat loss coefficient UL (Equation 11). UL = Qloss Ac(Tav − Tamb) = Ein − Qu Ac(Tav − Tamb) (11) Figure 10 and Figure 11 present the variation of the irradiance and the air flow rate on the two days of data collection. Figure 10. Test conditions: Airflow rate ṁ. Figure 11. Test conditions: Irradiance S. 3. Results The tests on the two systems were carried out on two different days at the same location and the pa- rameters describing the performance were measured and analysed for the two systems. The irradiance not only varied from one day to the other, but also varied throughout each day, and this affected the operation of the collectors. The irradiance reached higher val- ues for scenario A (with the reference SAH) than for scenario B (with the TE-SAH). 46 vol. 64 no. 1/2024 Solar air heating collector outdoor performance (a). Tamb and Tout (b). Tout − Tamb Figure 12. Temperature variations for the SAH. 3.1. Temperature variations Figure 12a presents the temperature variations for the inlet (ambient) and outlet air temperature for both scenarios. Ambient air, at temperature Tamb varying between 28 °C and 39 °C, flowed by natural convection through the collector to remove heat from the absorbing surface (PV and thermal) and exited the collector with a higher temperature. The maximum Tout was 68.2 °C in the TE-SAH, higher than 64.3 °C measured in the reference SAH. The temperature rise (Figure 12b) followed the pattern of the variations of solar irradiance throughout the day. During the sun peak-hours (11:00–15:00), Tout − Tamb were at their maximum. It can be seen from Figure 12b that the temperature rise was higher in the TE-SAH than in the reference SAH. The maximum Tout−Tamb value in the TE-SAH was 32 °C, 6 °C higher than that achieved in the reference SAH (26 °C). The energy produced by the PV module powered the TECs at the back of the absorber plate, to pump heat from it. The heat was dissipated into the air duct by the fins attached to the TE hot side. Thus, the flowing air gained more useful heat, resulting in higher values of Tout − Tamb for the TE-SAH than during the test on the reference SAH, and even though the incident irradiance was higher in the reference SAH than for the TE-SAH. After 15:00, when the solar irradiance started to decrease, the temperatures started to decrease too. 3.2. Thermal energy conversion The temperature rise, related to the air mass flow rate through the collector, was used to compute the thermal energy gain rate Qu of the collector in the two scenarios Equation 4. Figure 13 presents the input and output energy of the two collectors. The input energy was higher for the reference SAH than for the TE-SAH, due to higher irradiance during the test with the reference SAH (Figure 13a). The heat gain varied for both scenarios throughout the day, being affected by the variations of the irradiance and the airflow rate (Figure 10 and 11). However Figure 13b shows that Qu reached maximum values of 272.53 W at noon and 313.71 W at 13:00 for the reference SAH and the TE-SAH, respectively. After noon, the thermal energy gain was higher for the TE-SAH than for the reference SAH, depicting the additive effect of the TE heat pumping used in the TE-SAH. During the sun’s peak hours, the electrical output, hence the power input to the TECs, increased. Thus, the TECs pumped more heat, adding to the heat collection by the air in the TE-SAH. Figure 14 presents the PV energy output for the two scenarios and the power input to a single TEC in the TE-SAH. The power output of the PV module was at its peak between 11:00 and 15:00, with a value ranging between 6 and 8 W. Figure 15 shows that the cumulative heat gain Qcum of the TE-SAH was lower than that of the reference SAH between 9:00 and 13:00, whereafter Qcum values were higher for the TE-SAH than for the reference SAH. Thus, the TE-SAH had a higher thermal energy production rate than the reference SAH. The cumula- tive heat gain, for the duration of the experiment was 2.25 MJ for scenario B against 2.10 MJ for scenario A. Thus, the application of TE heat pumping contributed to a 7.14 % increase in the thermal energy production of the solar collector, despite the difference in the energy input. Furthermore, the thermal energy losses were deter- mined and compared for the two scenarios to evaluate the effect of TE heat pumping on the thermal losses of the air collector. Using Equation 11, the heat loss coefficient UL was calculated for both the reference and TE-integrated collectors. The average value of UL was 5.4 W m−2 K−1 and 3.4 W m−2 K−1 for the reference collector and the TE-integrated collector, respectively. Thus, the TE-SAH’s heat loss coefficient was also reduced compared to the reference SAH. Hence, correspondingly, the thermal heat losses were reduced due to the application of TE-heat pumping. 47 Josué R. Segnon, Howard O. Njoku Acta Polytechnica (a). Input energy Ein (b). Output energy Qu Figure 13. Energy flow for the SAH. Figure 14. Output energy of the PV module and energy input to the TECs. As shown in Figure 16a, the instantaneous values of heat loss Qloss were higher for the reference SAH than for the TE-SAH, during peak sun hours when the PV electrical output was channeled into the TECs to perform heat pumping from the absorber plate, thus also increasing the heat collection by the working air. The maximum values of Qloss were 55.1 W and 23.0 W for the reference SAH and TE-SAH, respec- tively. Figure 16b compares the cumulative thermal heat losses for the two collectors. It can be seen that the reference collector lost more thermal energy than the TE-SAH. Throughout the tests, the cumulative heat losses were evaluated at 0.39 MJ for the TE-SAH, and 0.85 MJ for the reference SAH. Thus, the thermal heat losses were reduced by 0.46 MJ, corresponding to a decrease of 54.12 %, due to the application of TE heat pumping. Also, as shown in Figure 17, the absorber plate temperatures for the TE-SAH reached lower values than for the reference SAH. This is the combined effect of the lower irradiance incident in the Figure 15. Cumulative thermal energy gain. case of the TE-SAH, compared to the reference SAH, and the application of the TE heat pumping. Thus, with PV-TE heat pumping, the electricity from the PV module is used by the TEC to pump heat from the absorber plate, and cooling it. Consequently, the lower temperatures of the absorber plate represent an important driver of the reduced heat losses observed in Figure 16. 3.3. Thermal energy efficiency Figure 18 compares the instantaneous energy efficien- cies of the solar collectors in the two scenarios. The energy efficiency was significantly increased with the application of TE heat pumping. Between 12:00 and 15:00, due to maximum irradiance, the PV output was at its maximum (Figure 14), powering the TECs which pumped heat from the absorber plate, increas- ing the temperature at the TE hot side, thus allowing the working air to acquire more useful heat, which in- creased the thermal efficiency of the system. Figure 19 compares the cumulative efficiencies of the SAH in 48 vol. 64 no. 1/2024 Solar air heating collector outdoor performance (a). Instantaneous (b). Cumulative Figure 16. Energy losses. Figure 17. Absorber plate temperatures. the two scenarios, throughout the experiments. At the end of the day, the cumulative efficiencies of the SAH were 28.03 % and 46.73 % for scenarios A and B, respectively, representing an increase of 66.71 % in energy efficiency. This shows that TE heat pumping also increased the energy efficiency of the collector. Also, using Equation 8–9, the heat removal factor was also estimated and compared for the two collectors. The average value of FR was 0.49 for the reference SAH, and 0.55 for the TE integrated SAH. Thus, with the application of TE heat pumping, the heat removal factor was also improved by 0.06, corresponding to an increase of 12.24 %. 4. Conclusions In conclusion, the integration of thermoelectric coolers (TECs) as heat pumps into a solar air heater (SAH) demonstrated notable improvements in performance parameters in our outdoor setup. By using the output energy from a photovoltaic (PV) module to power the TECs attached to the SAH absorber plate, we observed a significant increase in the outlet air tem- perature compared to a reference SAH, despite lower irradiance intensity. Specifically, the temperature rise in the TE heat pumping SAH surpassed that of the reference SAH. Figure 18. Energy efficiency. Figure 19. Cumulative energy efficiency. 49 Josué R. Segnon, Howard O. Njoku Acta Polytechnica Notably, although the input energy was lower for the TE-SAH, the energy output was improved, particu- larly during peak sunlight hours when the PV elec- trical output reached its maximum, enabling effective heat pumping by the TECs. As a result, the TE heat pumping system improved heat collection by 7.14 %. Furthermore, the energy efficiency of the TE heat pumping SAH reached 46.73 %, which is 66.73 % higher than the conventional SAH with an energy efficiency of 28.03 %. This improvement was accompa- nied by a reduction in heat loss coefficient and overall heat losses, leading to a decrease in heat losses by 0.46 MJ over the test duration. Additionally, the heat removal factor FR experienced an increase with the incorporation of TE heat pumping. It is crucial to note that the performance of the TE heat pumping system is intricately tied to the power input to the TECs, directly dependent on the PV electrical energy output. This study demonstrates the effectiveness of utilising TECs as heat pumps in solar air heaters, offering a more efficient and environmen- tally sustainable approach to improve heat collection and energy efficiency in solar thermal systems. 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