Shanghai office installs PVT—generating both electricity and heat year-round. Will it be sufficient for winter and summer needs? Here’s the real-world data!

Nowadays, everyone is promoting renewable energy, and photovoltaic panels are hardly a novelty anymore—many buildings have already installed them on their rooftops. However, photovoltaic panels have one major issue: they’re highly dependent on solar radiation. When the sun is strong, they generate plenty of electricity, but when sunlight weakens, their output drops significantly, compromising the stability of both electricity and heat supply for buildings.

Recently, I’ve been paying attention to the north-facing rooms in an office building in Shanghai, specifically studying how a PVT system can provide energy. You might not have heard of PVT—short for Photovoltaic-Thermal systems—but essentially, it builds on photovoltaic technology by also harnessing the heat generated by the solar panels. This allows the system to produce both electricity and thermal energy simultaneously, making it far more efficient than installing photovoltaic or solar thermal panels separately. Using numerical modeling, I calculated how much electricity and heat this system could generate over the course of a year. I also estimated the heating and cooling demands for this particular room—how much warmth would be needed in winter, and how much cooling in summer—and then compared these figures to see if they matched up. First, let me share some of the baseline data I used. I referred to typical meteorological data for Shanghai from that year, which recorded a maximum temperature of 36.8°C and a minimum of -4.48°C. Over the entire year, each square meter of the building received solar radiation energy equivalent to roughly 154 kilograms of standard coal burned—enough to power significant energy needs. As for seasonal divisions, I followed the commonly accepted pattern: spring spans from March 23 to May 30, totaling 69 days; summer runs from May 31 to October 3, lasting 126 days; autumn covers October 4 to December 5, with 63 days; and winter includes January 1 to March 22, as well as December 6 to December 31, adding up to 107 days. The office under study is located on the north side, measuring 86 square meters, with windows accounting for 37% of the total wall area. All the building envelope components—such as the walls and windows—were designed according to Shanghai’s energy-saving standards for public buildings. For instance, the windows are coated triple-glazed units with a thermal transmittance coefficient of 2.20 W/(㎡·K). ・ ℃); the exterior wall consists of reinforced concrete topped with gypsum board and then layered with polystyrene foam, achieving a thermal transmittance coefficient of 0.62 W/(㎡·K). ・ ℃). Indoor temperature control also requires precision: turn on the air conditioner to cool when the temperature exceeds 28℃, and maintain it at 26℃; switch on the heater when the temperature drops below 16℃, keeping it at 18℃. Additionally, ventilate the room with outdoor air twice every hour. I built a model using DeST software, inputting all these parameters to calculate the hourly cooling and heating load requirements for the entire year. Now, let’s talk about the PVT system. In winter, this system uses the hot water produced by the PVT collectors to heat the rooms, while simultaneously leveraging the electricity generated by the PVT modules to power the air conditioner, assisting in the heating process. In summer, the same electricity from the PVT system drives the air conditioner for cooling purposes. The air conditioner boasts a cooling energy efficiency ratio (EER) of 2.7 and a heating energy efficiency ratio (COP) of 2.55. The PVT components are flat-plate type, with an area identical to the room—86㎡—and consist of a glass cover plate, photovoltaic modules, a back panel, fluid channels, and insulation boards. When sunlight hits the glass cover plate, part of the energy passes through to the PV modules, which convert it into electricity. The remaining heat is either dissipated into the surrounding air or transferred via the back panel into the water circulating through the fluid channels. Meanwhile, the insulation boards help minimize heat loss, ensuring maximum efficiency.

I developed energy balance equations for each component of the PVT system and set up several key parameters. For instance, at 20°C, the photovoltaic module achieves an efficiency of 0.12 for converting sunlight into electricity, with efficiency dropping by 0.0045 for every 1°C increase in temperature. Meanwhile, the mass flow rate of water is 0.1 kg/s, and the inlet water temperature varies significantly between spring/winter and summer/autumn. Using a self-written program, I performed iterative calculations to determine critical parameters such as component temperatures, photoelectric conversion efficiencies, and outlet water temperatures from the heat exchanger channels. These data were then used to compute the overall electricity generation and heat production of the PVT system. First, let’s examine the building’s heating and cooling demands alongside the PVT system’s thermal and electrical outputs over the entire year. On an annual basis, the building’s heat demand far exceeds its cooling needs. Notably, heat demand peaks during late autumn through early spring, with a brief but noticeable spike in early spring—though only for four days does the heat demand surpass 40 kWh. In contrast, cooling demand remains relatively low throughout most of the summer, with nearly half the time seeing no cooling load at all. When cooling is required, the average daily demand hovers around 54.50 kWh. In autumn, heat demand rises compared to spring, peaking sharply over the final 23 days of the season. Winter, however, brings the highest heat demand, steadily increasing from around 70 kWh per day after winter begins, reaching a peak of 149 kWh on December 22nd before gradually declining to about 80 kWh by mid-January. The demand then rebounds again in late February, remaining above 110 kWh per day until the end of the month. On average, the building’s annual heat demand totals 11,538.81 kWh, while cooling demand stands at 3,217.44 kWh. Turning to the PVT system’s performance, its annual thermal output reaches 78,079.20 kWh, far exceeding the building’s heat demand, whereas annual electricity generation amounts to 7,838.88 kWh—nearly double the building’s cooling requirements. This indicates that, on an annual scale, the PVT system generates more thermal energy than the building consumes, though it slightly underproduces electricity relative to cooling needs. Now, let’s delve deeper into how well the PVT system matches the building’s energy demands across different seasons: - **Spring**: During this season, the building’s total heat demand is 478.26 kWh, with no cooling load. Meanwhile, the PVT system produces 1,901.93 kWh of electricity and generates 26,712.03 kWh of thermal energy. As a result, the system’s electricity supply ratio (the multiple by which it meets or exceeds demand) is 10.14, while the thermal supply ratio soars to 55.85—a clear indication of ample thermal capacity. - **Summer**: Here, the building has no heat demand but requires 3,217.40 kWh of cooling. The PVT system steps up, generating 3,164.13 kWh of electricity and producing 35,928.86 kWh of thermal energy. The electricity supply ratio dips slightly to 2.66, while the thermal supply ratio remains robust at 5.83, underscoring the system’s ability to meet both heating and cooling needs effectively. - **Autumn**: In this season, the building’s heat demand is 936.9 kWh, with no cooling requirement. The PVT system delivers 1,055.30 kWh of electricity and produces 5,465.39 kWh of thermal energy. The electricity supply ratio climbs to 2.87, while the thermal supply ratio holds steady at 5.83, demonstrating the system’s flexibility in balancing diverse energy demands. - **Winter**: Winter presents the greatest challenge, with the building requiring 10,123.64 kWh of heat but receiving just 1,717.52 kWh of electricity and 9,972.93 kWh of thermal energy from the PVT system. As a result, the electricity supply ratio plummets to a mere 0.43, while the thermal supply ratio stays at 0.99. Despite these lower ratios, the combined thermal-electrical approach still manages to satisfy the building’s heating needs on most days. Finally, examining the system’s performance on a daily basis reveals some interesting dynamics. During winter, PVT thermal output successfully meets or even exceeds heat demand on most days—except for 39 days when supply falls short, accounting for 36% of the season. Remarkably, on 25 of those days, the supply ratio drops below 0.5, representing 23% of the winter period. Moreover, there’s an inverse relationship between the building’s heat demand and the PVT system’s thermal output: on sunny days, when PVT generates abundant heat, the building’s demand tends to be lower; conversely, when demand spikes, PVT thermal production wanes. On the coldest day of winter, PVT thermal output was available only between 10 a.m. and 3 p.m., despite extended electricity generation hours. However, the limited thermal energy produced during this window wasn’t enough to fully meet the building’s instantaneous heating load, which peaked at 10 a.m. at 10 kW. Although the PVT system managed to accumulate about 6 kWh of cooling capacity by the time air conditioning started at 9 a.m., its initial cooling power of just 3 kW—less than a quarter of the peak cooling load—proved insufficient to keep up. Fortunately, as the day progressed, the PVT’s supply ratio improved significantly, reaching 0.87, 1.03, 0.49, and 1.08 at various intervals. Yet, by 7 p.m., the system had completely exhausted its cooling potential, leaving the building without adequate support for the remainder of the day. In stark contrast, on the hottest day of summer, the PVT system delivered a remarkably generous thermal output, comfortably surpassing the building’s cooling demand. On that particular day, the system provided sufficient cooling energy for six full hours—more than half of the building’s total cooling duration—highlighting its exceptional capability to handle peak summer loads.Spring and autumn are transitional seasons when heating demand in rooms is low, with PVT systems supplying less energy than needed for only 1 to 2 days. On typical days, the total energy supply is sufficient, though it initially falls short at the start of the heating season before eventually exceeding the load requirement. In spring, the system can provide continuous energy for up to 8 hours on a typical day, while in autumn, this duration drops to 6 hours.

From these data, several conclusions can be drawn. First, the thermal demand in this shaded office—both in terms of duration and magnitude—is significantly higher than the cooling demand; throughout the year, the heating load is 3.6 times greater than the cooling load. Second, the energy supply from the PVT modules exhibits distinct seasonal fluctuations: production during spring and summer is higher than in autumn and winter, with electricity generation roughly accounting for one-tenth of the heat produced. Third, on an annual basis, the energy supplied by the PVT system far exceeds the room's actual demand. However, when analyzed on a daily basis, 36% of winter days fail to meet the heating requirements, while 22% of summer days fall short of satisfying cooling needs. That said, the parameters used in this study were derived from an idealized database, so real-world conditions may differ. Moving forward, it’s essential to account for practical factors such as internal heat interference within the building. Additionally, we should refine the predictive model for PVT thermoelectric output and integrate energy storage systems into the setup. With energy storage in place, not only can we capture the excess electricity generated before the air conditioning system kicks in, but we can also store surplus heat for use during nighttime or cloudy periods. This approach should enhance the overall efficiency of matching the PVT system’s output with the building’s thermal load.