The IntegraTE program accompanies the fast-growing market for PVT heat pump systems. A core component of the program is to conduct surveys under real-world conditions to investigate the reliability and performance of heating systems. Important results from the multi-year monitoring of five single-family homes have now been published.
■ Monitoring of the PVT heat pump system shows that good efficiency values are also achieved when retrofitting in a typical existing building.
■ It is very important for the PVT collector and brine heat pump to form a well-coordinated overall system. The range of source temperature variation may be more comparable to that of an air-to-water heat pump.
■ A considerable part of the self-generated photovoltaic power can be directly used for the operation of the heat pump, without the need for "intermediate storage in the grid". Combined with the power storage system, the coverage can be improved.
The demand for PVT collectors is increasing. In the last four years, their sales in Germany have increased almost fivefold. In 2022, the sector had PVT collectors with an area of about 19 100 m2 installations. Photovoltaic thermal (PVT) collectors are considered an alternative to brine heat pumps for the development of geothermal sources.
PVT collectors generate electricity and heat from solar radiation and the environment and supply it to heat pumps to improve their efficiency and CO2 balance sheet. In this combination, the total available energy (i.e., thermal and electrical) produced by a PVT collector in a calendar year is approximately four times that of a PV system with the same area.
倡议 IntegraTE
In order to raise awareness of the technically and economically attractive energy supply provided by PVT collectors in combination with heat pumps in the construction sector, the Federal Ministry for Economic Affairs and Energy has been funding the IntegraTE program since the end of 2019. Since December 2019, three scientific partners have been working together with Fraunhofer ISE in Freiburg, the Institute for Building Energetics, Thermal Technology and Energy Storage (IGTE) at the University of Stuttgart, and the Institute for Solar Energy Research in Hamelin (ISFH). At the same time, 16 system suppliers from Germany and neighbouring countries joined the initiative.
One of IntegraTE's core activities is the measurement of PVT heat pump systems under real-world conditions. IntegraTE's industrial partners are able to propose model houses, which are then equipped with a wide range of measurement technologies.
"Such a young, dynamic industry needs scientific support," emphasises IGTE's research partner Harald Drück. A two-year survey has now been conducted on the first single-family homes with investment concepts from different system suppliers (Figure 2). Monitoring data is publicly available on the IntegraTE website pvt-energie.de.
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Figure 2: Single-family homes surveyed at a glance. The brief description of the project shows the heating technology and heating distribution system (underfloor heating or radiator) in the house. The number corresponds to the system number in the publicly accessible monitoring portal pvt-energie.de. The results of House 1 are shaded because the gas boiler is installed parallel to the heat pump, so the heat pump is more likely to operate in favorable weather conditions with a priority control system.
JAZ as a key parameter
The core parameter of the evaluation system is the Annual Performance Factor (JAZ). It is defined as the amount of heat provided annually for space heating and potable water heating, in relation to the electrical energy required for control and pumping in heat pumps (compressors, heating elements, and controls) as well as in heat source circuits.
Korbinian Kramer, IntegraTE Project Manager at Fraunhofer ISE, says: "We did not factor the heat storage of the house into the system limits of the annual coefficient of performance, so the different heating systems had little impact on the operating results, and we achieved greater comparability. In addition, Figure 2 shows the JAZ for purely heating operations. Here, the share of solar energy from PVT collectors that are directly used to operate the heat pump is not deducted.
Altenbuch's single-family residential building has a JAZ of 4.2, which is the best performer (house 5). Here, the PVT collector works with three underground baskets in the garden to supply heat to the heat pump. In the two measurement years, the JAZ was 3.3 and 3.4, respectively, and the large renovated single-family house (House 3) in Sweden also achieved satisfactory efficiency values. Project manager Kramer emphasizes that all demo systems, both new and old, have been fairly stable for the first two years. Thanks to the exchange with the specialized companies that carry out the work, it is also possible to optimize some systems in terms of energy efficiency.
The results of the operation of the two factories will be studied in more detail below as examples of the new building (Building 4) and the old building (Building 3).
Between 10% and 40% of PVT electricity can be used directly by the heat pump system
The newly built residential building in Hasfeld, Lower Saxony (house No. 4, 190 m2 of living space) is heated only by a 6 kW PVT collector controlled by a Th-brine/water heat pump. Next to the roof, a 16-m2-PVT field is installed, a 3.6 kW Ayer 1.8 kW pure photovoltaic system Ayer is installed.
For house 4, where a family of four lives, monitoring confirms that the annual performance figure is well over 3. This balance includes the power requirement for a 7 kW electric heating element, which, however, is rarely activated according to the monthly balance sheet for 2022 (Figure 3). In 2022, the heating system required a total of 2566 kWh of electricity. Of this, 79% is used for heat pumps, 13% is used for auxiliary power supplies for pumps and controls, and 8% of the energy is converted directly into heat in the heating element.
Kramer: In operation, a PVT heat pump heating system with a PVT collector as the sole heat source for the heat pump achieves similar efficiencies to a system with PVT and a ground collector, although the cost of purchasing the heating element is much cheaper than that of a ground collector.
Figure 3 shows how the efficiency of the heating system, represented by the monthly coefficient of performance of the heat pump, changes over year. It can also be seen that the high temperature of drinking water heating, which dominates in summer, is often generated by heat pumps, which is less efficient than the medium heating water temperature that dominates the thermal equilibrium in autumn and winter.
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Figure 3 Monthly energy balance for house 4 – A newly built single-family house in Hasfeld, Lower Saxony, has a heat pump that draws heat only from a PVT collector field and is protected by an electric heating element as additional heating.
During the monitoring process, the temperature in the solar circuit of the PVT collector was also recorded. Varies depending on the season. In January and February, the energy-weighted initial temperature of the PVT collector during that month was 1.7 °C and 0.4 °C, respectively. On some days, PVT collectors have expansion temperatures of up to -12°C in winter. Therefore, the pipes in the solar circuit from the roof to the heat pump must be well insulated within the building envelope to avoid the formation of condensation on the outside of the insulation.
In July and August, the average temperature of the solar circuit is 18.4 °C and 19 °C, respectively. Heat pumps must be designed for this wide range of source temperatures. Therefore, the PVT heat pump system must always be a well-coordinated overall system, Kramer emphasises.
How does the annual coefficient of performance increase if the share of solar energy generated at the same time as the heat pump operation is subtracted? According to monitoring, with the help of batteries, the electricity from PVT collectors and additionally installed photovoltaic modules covers 33% of the annual electricity demand for heat pumps and heating elements in single-family homes in Hasfield. As a result, the annual performance coefficient adjusted for this self-consumption increased from 3.9 to 5.9.
Monitoring data from demonstration homes shows that in the absence of batteries, about 10 to 30 percent of the electricity generated in a reasonably sized photovoltaic system is directly used for heating purposes. After the installation of the storage system, this coverage increases to 25 to 40 percent.
The regeneration of geothermal probes increases the efficiency of the soil as a heat source
Exhibit 3 of the monitoring is a large, renovated single-family house in Sweden with an area of 340 m2 of living space. Action is needed here, as geothermal probes – operating for many years as the sole heat source for brine/water heat pumps – have significantly cooled the soil. If a significant "cold funnel" is formed around the geothermal probe, the probe's heat production rate will continue to decrease.
The PVT system installed in 2020 has an area of 31 m2 and a power of 19 kW Ale, so it fulfills three tasks at the same time: it supplies and heats directly to the brine/water heat pump (the yellow column section above the x-axis in Figure 4). Excess heat from the summer can also be fed into a geothermal probe and regenerated from there. Especially between May and September, the PVT system transports a large part of the heat to the ground (orange-brown columns below the x-axis). The monthly equilibrium of heat also shows an interesting effect, i.e., even in summer, when there is no heat directly from the PVT collector, the heat pump enters the ground (the brown bar section above the X-axis).
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Figure 4: Monthly heat source heat balance for an 11 kW brine/water heat pump in House 3 – a renovated detached house in Sweden with 340 m2 of heated living space. The heat from the PVT system and the ground flows directly to the heat pump and is displayed in a positive direction (above the x-axis). Excess PVT heat fed into the ground through the probe appears below the x-axis.
What is the overall balance to meet the heat demand for heating and hot water in residential buildings in Sweden? 25% of the source heat of the heat pump comes directly from the PVT collector field and 75% is supplied by geothermal probes. 76% of the source heat from the geothermal probe originally comes from the PVT system. Kramer: "This demonstration house shows that it is very helpful to supplement the existing geothermal detection field in an old building with a PVT collector field, because then the geothermal source can maintain its efficiency for many years without having to expand or even renew it.
In the case of a single-family house in Sweden, measurement technology shows that PVT electricity directly meets 21% (1427 kWh) of heat pump electricity demand during the year. House 3 does not have solar cells. In 2022, the annual working hours increased from 3.4 to 4.3 hours, if the impact of direct electricity consumption from photovoltaics is taken into account.
At higher heating circuit temperatures, the annual performance factor decreases
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Figure 5: Annual performance factors for existing single-family residential heat pump systems with and without PVT measured in 2019. The blue dots represent air source heat pumps, and the brown dots represent brine heat pumps. Both are part of the WP smart im Bestand project. The black dots show the five residential buildings surveyed by IntegraTE with data for 2022.
Figure 5 combines the results of field tests of a heat pump system without PVT from a previous monitoring project with the results of the new IntegaTE demo house. Each blue dot represents an air-water heat pump and each brown dot represents a saltwater-water heat pump – in an existing single-family home. The black spots of the five IntegraTE systems investigated were roughly between the efficiency of air/water and brine/water heat pumps. Kramer: "It's a great result, because the PVT heat pump system is a younger, less mature building technology, so there's definitely a lot of potential for improvement in the future."
Even though Figure 5 reflects the fact that the annual coefficient of performance of the heat pump depends on the flow temperature in the heating circuit and when heating drinking water, as expected, it can be seen that good efficiency values are also achieved in older buildings. "Based on our findings, PVT heat pump systems are also ideal for use in renovations," Kramer concluded.