Unglazed Photovoltaic-thermal Collectors Research Articles

Developing an Advanced PVT System for Sustainable Domestic Hot Water Supply

Energy consumption is steadily increasing with the ever-growing population, leading to a rise in global warming. Building energy consumption is one of the major sources of global warming, which can be controlled with renewable energy installations. This paper deals with an advanced evacuated hybrid solar photovoltaic–thermal collector (PVT) for simultaneous production of electricity and domestic hot water (DHW) with lower carbon emissions. Most PVT projects focus on increasing electricity production by cooling the photovoltaic (PV). However, in this research, increasing thermal efficiency is investigated through vacuum glass tube encapsulation. The required area for conventional unglazed PVT systems varies between 1.6–2 times of solar thermal collectors for similar thermal output. In the case of encapsulation, the required area can decrease by minimizing convective losses from the system. Surprisingly, the electrical efficiency was not decreased by encapsulating the PVT system. The performance of evacuated PVT is compared to glazed and unglazed PVTs, and the result shows a 40% increase in thermal performance with the proposed system. All three systems are simulated in ANSYS 18.1 (Canonsburg, PA, USA) at different mass flow rates and solar irradiance.

Performance evaluation and parametric analysis of an integrated diurnal and nocturnal cooling system driven by photovoltaic-thermal collectors with switchable film insulation

Cooling systems integrated with photovoltaic-thermal (PVT) collectors are promising for reducing building energy consumption because PVT collectors can provide photovoltaic power generation, solar thermal cooling, and nocturnal passive cooling. Glazed and unglazed PVT collectors have typically been used in previous studies, but they lead to conflicting goals for enhancing the all-day energy performance. Glazed collectors are favorable for solar thermal cooling but result in low nocturnal passive cooling capacity, while unglazed collectors exhibit the opposite behavior. In this study, PVT collectors with switchable film insulation (i.e., PVTfilm collectors) are introduced to drive an integrated diurnal and nocturnal cooling system. Such collectors can adapt to daytime and nighttime operating modes by inflating and deflating their air cushion. Annual (8760-hour) simulations were performed based on the case study of a tropical building. Three system layouts driven by different PVT collectors were compared. The effects of key optical parameters on energy performance were analyzed. The results show that the PVTfilm collectors combine satisfactory solar thermal cooling and nocturnal passive cooling performances, with an annual total energy yield that is 10% and 32% higher than those of glazed and unglazed PVT collectors, respectively. To improve the annual total electricity savings, it is advantageous to increase the film transmittance for solar radiation, decrease the film transmittance for long-wave thermal radiation and decrease the photovoltaic panel emissivity for long-wave thermal radiation. This work is relevant for PVT systems to improve the all-day energy performance and reduce building energy consumption.

Assessment of Electrical and Thermal Performance of Photovoltaic Thermal Air Collector in the Climate of Pantnagar, Uttarakhand

The photovoltaic thermal collector (PVT) produces electricity and extracts heat from a common area exposed to solar radiation. The present work provides a methodology to assess the potential of PVT at any location. The climate of Pantnagar (a town situated in the Terai belt of Uttarakhand) is selected for the study. An unglazed PVT air heating collector is modelled, taking account of both electrical and thermal aspects of design. The performance of the collector is then simulated on four days, representing average weather of four seasons observed in North Indian Plains, in correspondence with the typical meteorological year (TMY) data of 2006-2015 for the location. The diode parameters and characteristic curves of PV module selected for PVT are obtained for reference and operating conditions. The operating temperature of solar cell and heated air are plotted after convergence of numerical solution at available data points. Finally, electrical and thermal power output of the PVT with associated optical and thermal losses are obtained. The highest electrical power output of 223 W (136 W/m2) is observed at 12:20 hours (IST) on equinox of September while highest thermal power output of 411 W (251 W/m2) is obtained at 12:20 hours (IST) on summer solstice.

3-D numerical modelling and experimental investigation of coupled photovoltaic thermal and flat plate collector

Photovoltaic Thermal (PVT) systems generate hot water and electricity simultaneously. The grade of thermal energy generated by such PVT systems is low, resulting in limited application. To overcome this challenge, the coupling of the PVT system to a secondary flat plate collector (FPC) has been explored in this work. For the system's performance estimation, a 3-D numerical model has been developed for both glazed and unglazed PVT-FPC collector. Based on the numerical model, PVT collectors have been fabricated, and each is then connected in series to a commercially available FPC collector. Experiments conducted on the fabricated PVT-FPC system showed a close match between simulation and experiments. Further, experiments conducted on the unglazed PVT – FPC collector showed a peak outlet water temperature of 60–63 °C at 30 LPH. For the unglazed PVT collector, the peak electrical and thermal efficiency of 16% and 25% is reported respectively, whereas for the FPC collector, the thermal efficiency of 35% is reported. Compared to an individual unglazed PVT collector, 17 °C higher outlet water temperature is reported for the coupled system. Similarly, for the case of glazed PVT-FPC collector, the peak outlet water temperature reported is 65–67 °C at 30 LPH with 14–15 °C higher outlet water temperature.

Evaluation and comparison of solar trigeneration systems based on photovoltaic thermal collectors for subtropical climates

Solar trigeneration systems based on photovoltaic thermal (PVT) collectors are promising for enhancement of solar energy utilization. Single-effect LiBr-H2O absorption chillers are usually employed in these systems. Since the temperature of solar heat from PVT collectors is low, half-effect absorption chillers are potential alternatives for performance improvement. However, the coefficient of performance (COP) of half-effect absorption chillers is lower than that of single-effect machines. Therefore, the comparison of solar trigeneration systems with both types of chillers is necessary. Four different system layouts, obtained by coupling glazed and unglazed flat-plate PVT collectors with single- and half-effect absorption chillers, are modeled. Annual (8760-hour) simulations are performed to evaluate and compare the thermodynamic and economic performances of different layouts. The results show that the system layout based on glazed PVT collectors coupled with half-effect absorption chillers achieves the highest Solar COP of 0.072 and the highest solar utilization factor of 0.241. Its specific electricity saving is 158.5 kWh/(m2·year), the highest among all the layouts. Additionally, the payback period of the layout based on unglazed PVT collectors coupled with half-effect absorption chillers is 12.7 years, the lowest among all the layouts. This paper is helpful for the development and design of solar trigeneration systems based on PVT collectors in subtropical climates.

Performance of Unglazed Photovoltaic-Thermal Collectors for Cooling Purpose

Unglazed photovoltaic-thermal (PVT) collectors for combined usable heat and electricity production are still expanding on the market. Besides the summer heat production and whole year electricity production the unglazed PVT collectors can be used also for night cooling (sky radiation and wind convection heat exchange). Different unglazed PVT collectors have been experimentally tested for verification of reliable model for night cooling operation. Based on the results, the model has been implemented into simulation software TRNSYS. Simulation analyses of the cooling performance for different climate and operation conditions have been performed. While best of tested unglazed PVT collectors can achieve average specific cooling power around 110 W/m2 (80 kWh/m2.season) at chilled water temperature 20 °C in the climate conditions of central and northern Europe, in the climate conditions of south Europe these unglazed PVT collectors achieve not more than 38 W/m2 (27 kWh/m2.season) during summer season (June–August).

Numerical model for the thermal yield estimation of unglazed photovoltaic-thermal collectors using indoor solar simulator testing

Abstract It is a common practice to test solar thermal and photovoltaic-thermal (PVT) collectors outdoors. This requires testing over several weeks to account for different weather conditions encountered throughout the year, which is costly and time consuming. The outcome of these tests is an estimation of the thermal performance characteristics of the collector. Collector performance parameters can be derived with less effort by indoor testing under a solar simulator. However, in case of unglazed PVT collectors the thermal and the electrical performance is affected by two phenomena-additional long wave radiation (3000 nm and greater) due to emissions and reflections from the high temperature artificial sky, and an energy content of the PV spectrum (300–1100 nm) that differs from the global solar spectrum (300–2500 nm). These differences from the reference AM 1.5 solar spectrum lead to errors in the estimation of collector thermal and electrical performance. Therefore, results of indoor performance tests must be corrected to obtain the output of an unglazed PVT collector in real outdoor environment. In this paper a method is proposed to estimate the real thermal performance of unglazed PVT collectors, by using a compact indoor solar simulator testing in combination with a detailed steady state numerical PVT collector model. The numerical model takes into account the physical and spectral attributes of the solar simulator and is used to correct for the unwanted phenomena to derive the actual outdoor collector performance. The resulting numerical model also offers detailed understanding of the collector and can therefore be used to optimise the design of the collector. Furthermore, this model is used to derive thermal performance characteristics of the unglazed PVT collector as defined by solar thermal testing standards, which can be used in system simulation tools (E.g. TRNSYS models) to obtain annual collector and system yields.

A PVT Collector Concept with Variable Film Insulation and Low-emissivity Coating

Hybrid photovoltaic-thermal (PVT) collectors co-generate solar electricity and heat in a single component with an optimum utilization of space. Spectrally selective but transparent low-emissivity (low-e) coatings are a proven method to reduce thermal losses. However, overheating and stagnation are critical issues for these collectors due to material degradation, thermal stress, and low electrical efficiency.This paper presents a PVT collector concept with variable film insulation as overheating protection. An inflatable glass-film cushion regulates thermal losses. Performance and stagnation tests were carried out with a prototype. During normal operation the collector achieves a high thermal efficiency. In periods of standstill, the deflated cushion has a high heat dissipation rate by deactivating the low-e coating. Stagnation temperatures are thus limited to 95 ̊C. To conclude, the PVT collector combines the advantages of glazed and unglazed PVT collectors which are a high thermal efficiency and low stagnation temperatures.

Unglazed PVT collectors as additional heat source in heat pump systems with borehole heat exchanger

An unglazed photovoltaic-thermal (PVT) collector provides a double benefit in a heat pump system compared to a conventional heat pump system with PV and borehole heat exchanger. First, the PV cells in the PVT collector are cooled, leading to lower cell temperatures and a higher PV efficiency. Second, the PVT heat raises the temperature level of the heat pump and the borehole heat exchanger and consequently leads to a higher heat pump system performance. In this context a system was measured with a 39 m2 unglazed PVT collector field. The impact of the solar heat to the heat pump is determined by TRNSYS simulations using energetic weighted temperatures for validation and to quantify the impact of the solar heat. The investigated system revealed a temperature increase of 3K which equals 9% lower electricity consumption. The yearly additional PV yield was determined to 4% by reference measurement of uncooled conventional PV panels. Additionally, system simulations in TRNSYS are presented.

Comparison of Electrical and Thermal Performances of Glazed and Unglazed PVT Collectors

Photovoltaic-thermal (PVT) collectors combine photovoltaic modules and solar thermal collectors, forming a single device that receives solar radiation and produces electricity and heat simultaneously. PVT collectors can produce more energy per unit surface area than side-by-side PV modules and solar thermal collectors. There are two types of liquid-type flat-plate PVT collectors, depending on the existence of glass cover over PV module: glass-covered (glazed) PVT collectors, which produce relatively more thermal energy but have lower electrical yield, and uncovered (unglazed) PVT collectors, which have relatively lower thermal energy with somewhat higher electrical performance. In this paper, the experimental performance of two types of liquid-type PVT collectors, glazed and unglazed, was analyzed. The electrical and thermal performances of the PVT collectors were measured in outdoor conditions, and the results were compared. The results show that the thermal efficiency of the glazed PVT collector is higher than that of the unglazed PVT collector, but the unglazed collector had higher electrical efficiency than the glazed collector. The overall energy performance of the collectors was compared by combining the values of the average thermal and electrical efficiency.