Field Assessment Tests Modular Liquid Photovoltaic Thermal System Performance in Korea
Researchers Minseo Park, Hooseung Na, Sungwoong Yang, and Su-Gwang Jeong evaluated the electrical and thermal operating characteristics of a modular liquid-based photovoltaic and thermal system under real-world outdoor operating conditions. Conducting their investigation at a field test facility operated by Janghan Technology Co., Ltd. in Dangjin-si, Chungcheongnam-do, Republic of Korea, the team analyzed two continuous weeks of field monitoring data gathered at fifteen-minute intervals between June 15 and June 28, 2025. Over the fourteen-day outdoor evaluation, the time-of-day averaged electrical efficiency of the modular installation ranged from 5.18 percent to 5.89 percent, with a diurnal mean value of 5.44 percent. Concurrently, the time-of-day averaged thermal efficiency spanned from negative 5.30 percent to positive 13.38 percent, yielding an overall daytime mean value of 4.06 percent. The authors found that while electricity generation correlated strongly with incident solar radiation, useful heat recovery depended substantially upon the thermal state of the closed-loop system storage tank, demonstrating how real outdoor operational dynamics diverge from laboratory conditions.
The investigation addresses broader decarbonization efforts within the building sector, which accounts for a substantial share of global final energy consumption and greenhouse gas emissions. While conventional photovoltaic panels are widely deployed due to ease of installation and low maintenance requirements, standard crystalline silicon cells convert only approximately fifteen to twenty percent of incident solar radiation into electrical power. The remaining solar energy converts into heat that accumulates within the module, driving up operating temperatures and degrading electrical conversion efficiency. Photovoltaic and thermal systems combine solar photovoltaic generation with thermal collectors in a unified system, recovering heat from the rear surface to supply usable hot water or space heating while dampening module heating. However, most previous studies have relied on numerical simulations or controlled laboratory experiments that fail to capture the simultaneous interactions of shifting weather, variable fluid temperatures, and continuous thermal storage in operating buildings.
To evaluate practical applicability and ease of retrofitting, the experimental installation tested a modular architecture where a liquid heat exchanger was attached directly to the rear surface of a standard commercial photovoltaic module. This independent manufacturing and assembly approach permits the same heat recovery hardware to be applied across diverse commercial photovoltaic panels without requiring specialized composite lamination or permanent adhesive bonding layers. The commercial photovoltaic module utilized monocrystalline silicon solar cells with a rated electrical output of 275 watts and dimensions of 1,675 millimeters by 1,001 millimeters by 115 millimeters, mounted due south at a tilt angle of 31.3 degrees. The rear liquid thermal collector incorporated larger header pipes connected by multiple small-diameter parallel flow channels to circulate heat-transfer fluid, backed by a rear insulation layer to limit heat loss. Specific material compositions of the flow channels and the precise thickness and type of insulation were not preserved in the facility archives.
The experimental apparatus operated as a closed loop consisting of the photovoltaic-thermal module, a dedicated thermal storage tank, a circulation pump, connecting distribution pipes, and comprehensive monitoring sensors. To ensure freeze protection during winter and maintain operational stability, the circulating fluid was a water-propylene glycol mixture containing forty percent propylene glycol by weight. Facility documentation recorded a fluid density of 1.063 kilograms per liter at ten degrees Celsius and a specific heat capacity of 0.89 kilocalories per kilogram per degree Celsius, equivalent to 3.72 kilojoules per kilogram-kelvin. Data were recorded at fifteen-minute intervals using a Yokogawa GX10 paperless data recorder, tracking solar irradiance, ambient temperature, wind speed, collector surface temperature, fluid inlet and outlet temperatures, flow rate, voltage, and current. Nonpositive and abnormally low flow readings below 0.5 liters per minute were excluded as invalid measurements, yielding an average circulation flow rate of 0.96 liters per minute ranging between 0.83 and 1.03 liters per minute.
Electrical power output was determined directly from measured operating voltage and current, while electrical efficiency was calculated as the ratio of generated power to incident solar irradiance across the module aperture area. Because historical documentation confirming maximum-power-point tracking or the specific electrical load configuration was unavailable, the authors noted that reported figures reflect the monitored operating point rather than rated conversion limits or maximum power point efficiency. Useful thermal power was calculated from mass flow rate, fluid heat capacity, and the temperature difference between outlet and inlet ports, with thermal efficiency defined as useful thermal power divided by incident solar energy. Efficiencies were computed for each fifteen-minute interval, and diurnal profiles were derived by taking arithmetic averages of identical clock-time values across the fourteen days rather than summing cumulative daily energy yields, avoiding distortion from incomplete flow records during certain measurement intervals.
Analysis of electrical performance demonstrated that electrical power output followed a typical diurnal trajectory governed by solar altitude. Generated power increased through the morning, reached its daily peak near solar noon alongside peak plane-of-array irradiance, and decreased across the afternoon. Linear regression between plane-of-array irradiance and electrical power output produced the equation P equals 2.289 plus 0.833 multiplied by irradiance, achieving a coefficient of determination of 0.944 that confirmed solar radiation as the dominant driver of electricity generation. Scattered departures from the regression line reflected fluctuating ambient temperatures, wind gusts, transient cloud cover, and module operating temperatures. In contrast to power output, electrical efficiency did not scale proportionally with irradiance, remaining within a narrow band of 5.18 percent to 5.89 percent and reaching its maximum between 16:00 and 17:00 without exhibiting an afternoon monotonic decline.
Thermal performance displayed distinct operating behavior governed by the thermodynamic condition of the closed-loop system. Useful thermal power rose during the morning as solar irradiance increased, reaching a daily peak around 10:30 before declining through the afternoon. As continuous circulation returned recovered heat to the storage tank, fluid temperatures inside the reservoir steadily climbed across daytime operation. This accumulation of stored heat elevated the inlet fluid temperature supplied to the collector module during the afternoon. Consequently, the temperature difference between the outlet and inlet streams diminished, progressively reducing the useful thermal power absorbed by the working fluid under comparable irradiance levels. This dynamic resulted in time-of-day averaged thermal efficiency peaking during morning hours and steadily declining thereafter, confirming that the thermal state of the storage tank represents a primary operational factor governing heat recovery in closed-loop configurations.
During several afternoon intervals, the calculated thermal efficiency dropped into negative territory, reaching values as low as negative 5.30 percent. This phenomenon occurred when declining afternoon solar irradiance coincided with an elevated storage tank temperature, causing incoming fluid to enter the collector at a higher temperature than the exiting fluid. Under these circumstances, circulating fluid experienced a net temperature drop across the module, indicating periods where the fluid stream released heat rather than harvesting thermal energy. Although the authors cautioned that lack of sensor-specific calibration logs prevented formal uncertainty propagation for values near zero, the occurrence of negative thermal differentials highlights practical challenges in closed-loop operation. Specifically, continuous pumping when inlet temperatures match or exceed outlet temperatures provides no useful heat extraction. These observations suggest that implementing differential-temperature pump control strategies based on inlet-outlet temperature differences could enhance operational performance, though evaluating such control logic remains an objective for future investigations.
Examining the relationship between inlet fluid temperature and thermal efficiency under irradiance levels of at least 200 watts per square meter revealed substantial scatter without any clear linear trend. The dataset also displayed repeated zero thermal-efficiency readings across the entire inlet temperature range despite confirmed active circulation, an observation whose exact operational origin could not be resolved from archived logs. Contextual comparison with prior literature revealed that the measured mean electrical efficiency of 5.44 percent and mean thermal efficiency of 4.06 percent were noticeably lower than figures reported in earlier liquid-based studies, where thermal efficiencies frequently exceeded 40 percent. The researchers explained that previous benchmarks were largely obtained under controlled laboratory conditions, steady artificial illumination, low inlet fluid temperatures, and optimal electrical loads, whereas the present field trial reflected continuous outdoor variations, heat buildup in the storage circuit, and unoptimized operating points.
Thermal recovery was also influenced by the mechanical configuration of the modular heat exchanger, which was installed in direct physical contact with the photovoltaic backsheet without thermal bonding adhesives. Interfacial thermal contact resistance across this dry physical junction likely restricted conductive heat transfer into the circulating fluid, although its precise contribution could not be isolated experimentally. Nevertheless, the modular approach offers practical benefits for building deployment, including compatibility with standard commercial modules and the ability to reuse thermal collectors when replacing aging photovoltaic panels. Study limitations included a two-week summer monitoring duration, a single test installation, the absence of an uncooled reference panel to isolate cooling-induced electrical benefits, and missing sensor calibration records. The authors concluded that future research should focus on multi-season monitoring, direct comparisons against reference photovoltaic modules, detailed quantification of interfacial contact resistance, and the development of differential-temperature control strategies for closed-loop building applications.
Subject of Research: Field performance evaluation of a modular rear-mounted liquid photovoltaic-thermal system
Article Title: Operational Assessment of a Modular Rear-Mounted Liquid-Based Photovoltaic/Thermal System under Field Conditions
Article References: Park, M., Na, H., Yang, S., & Jeong, S.-G. (2026). Operational Assessment of a Modular Rear-Mounted Liquid-Based Photovoltaic/Thermal System under Field Conditions. Architectural Research, 28(1), Article 30. https://doi.org/10.1007/s44515-026-00033-x
Image Credits: AI Generated
DOI: 10.1007/s44515-026-00033-x
Keywords: photovoltaic-thermal systems, solar energy, building energy, field monitoring, thermal efficiency, electrical efficiency, heat recovery, renewable energy, Operational, Assessment, Modular, Rear-Mounted
News Source: Faith Mcneil. (November 9, 2026). Field Assessment Tests Modular Liquid Photovoltaic Thermal System Performance in Korea. Scienmag.
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