PVT Wrap-Up – Energy systems with photovoltaicthermal solar collectors

Solar radiation that reaches a photovoltaic cell is only partially converted into electricity, whereas a large proportion of the solar irradiation is converted into heat. Photovoltaic-thermal (PVT) solar collectors (also referred to as “hybrid collectors”) enable the collection and thus utilisation of this heat. PVT collectors thus produce solar electricity as well as solar heat on the same surface and attain high surface-related yield levels.


There are various types of PVT collectors. A distinction is made between collectors with and without transparent front cover for reducing heat losses (covered / uncovered). A distinction is also made between collectors from which heat is extracted via a heat transfer liquid or via air as a heat carrier medium.


The market overview carried out within the framework of this study identified 53 products, the majority of which (38) are uncovered liquid-cooled PVT collectors. The 5 Swiss manufacturers only produce this type of PVT collector. There are currently very few covered collectors on the market. The dissemination of uncovered PVT collectors with heat extraction by air is on the increase.


In Switzerland, around 300 PVT systems are currently in operation with a total surface area of approximately 15,000 m², almost all of which are equipped with uncovered liquid-cooled collectors. At present around 3,000 m² of PVT collector surface is installed per year in Switzerland. By way of comparison, around 100,000 m² of purely thermal solar collectors and approximately 2,200,000 m² of photovoltaic modules are installed each year.


PVT collectors are used on the one hand in conventional areas of solar thermal energy such as hot water (pre)heating systems (~30% of the systems in Switzerland) and systems for hot water heating plus space heating support (~15% of the systems in Switzerland). Uncovered liquid-cooled PVT collectors are however also specially used in heat pump systems where the low-temperature heat is primarily utilised on the source side of the heat pump. Particularly worthy of mention is the regeneration of geothermal boreholes (~30 percent of the systems in Switzerland).


Seven largish PVT systems with collector surface areas between 50 and around 3,500 m² were developed as part of, or in the framework of, flagship or pilot and demonstration projects promoted by the Swiss Federal Office of Energy (SFOE). In these systems the use of PVT collectors for geothermal borehole (ground source) regeneration, the preheating of groundwater in combination with heat pumps, and their use in combination with an anergy (low-temperature) network was studied. All these systems are closely monitored.


The magnitude of the annual solar yields of uncovered PVT collectors (with a low inclination angle ~10°) in Switzerland’s central plateau is around 160 kWh/(m²a) (electricity) plus a thermal yield of ~150 kWh/(m²a) (hot water), ~250 kWh/(m²a) (hot water preheating) and 300-400 kWh/(m²a) (ground source regeneration or preheating of groundwater). With systems for hot water preheating and those for ground source regeneration, thanks to module cooling, electricity yields about 5% higher in comparison with basic PV modules are obtained.


Around 20% of the approximately 600 contacted solar technology companies participated in the survey concerning PVT systems. Roughly 80% of the companies have not yet realised PVT systems, while the remainder were already involved in the realisation of one or more systems. The majority of the companies are interested in PVT systems and have a fundamentally positive view of the technology. The companies see challenges as well as improvement potential with respect to the increased use of PVT systems, especially relating to economic viability, know-how, familiarity with the technology and product design.


The report makes the following conclusions in particular:


- Fully developed products in the area of uncovered, liquid-cooled PVT collectors are available and can be successfully put into operation.

- Other types such as covered PVT collectors or air PVT collectors are not yet widely available and some of them need further technological development.

- Well-conceived system integration and control is essential, especially for uncovered collectors, in order to ensure that the collectors are operated at low temperatures, and thus efficiently.

- Know-how regarding the use of PVT collectors exists, but it needs to be more widely distributed.

- The implementation of PVT systems requires cooperation between various trades (photovoltaics specialists, solar thermal energy specialists, roofers, etc.). To ensure that this does not represent an obstacle, the necessary cooperation should be promoted in a targeted manner.

- Legal provisions stipulating a minimum proportion of renewable energy in buildings open up an opportunity for PVT technology.

D. Zenhäusern, E. Bamberger, A. Baggenstos, 2017
SPF Institute for Solar Technology, Rapperswil