PV Thermal Systems — Capturing the Untapped Energy J. Hollick

Conserval Engineering Inc., 200 Wildcat Road, Toronto Ontario Canada M3J 2N5
Corresponding Author, ihollick@solarwall. com


Canada’s National Solar Test Facility and the Danish Technological Institute have completed testing of PV Thermal modules as part of the International Energy Agency Task 35 Project. The data shows that it is possible to capture two to three times more thermal energy than electricity from a PV array. Panels from various manufacturers were tested under NOCT conditions, and the results showed that when PV modules were mounted on top of SolarWall® transpired collector panels, the total solar efficiency was in the 25% to 50% range depending on the PV module tested, compared to the typical 6 to 12% for PV modules alone.

By removing the excess heat generated by the PV modules, the electrical output is increased. Modules can commonly operate at temperatures over 50 degrees above ambient temperature resulting in a performance reduction of more than 25%. By removing the heat from the module and lowering the operating temperature, significant gains can be made in system performance and the heat can be utilized for practical heating purposes. The economics of a PV system that incorporates a thermal component can also be improved on buildings where the PV heat can be used to displace space heating energy.

Keywords: PV thermal, transpired collector, BIPV

1. Introduction

The trend with photovoltaic (PV) installations is towards building integrated systems, and while this is advantageous in many regards, there are problems associated with conventional methods of integrating PV directly into a building.

The main problem with building integrated photovoltaic (BIPV) systems is heat retention under the PV modules. The heat produced can be as much as 50°K (90°F) over ambient temperature resulting in two concerns. The first is the possible structural damage from heat if panels are not vented or if heat is not recovered. The second is the lower efficiency of most PV modules with increasing temperature. Crystalline cells are affected by temperature and the performance drops as cell temperature rises. It has been shown that for each °C increase in temperature, the power production drops by ~0.5%. This means that a BIPV 100 W crystalline module at 65°C is only delivering 80 W of power compared with the 25°C name plate rating.

A PV module operating at its stagnation temperature of 50°K above ambient, when the heat is not removed and if ambient temperature is 30°C, will experience a module temperature of 80°C, or even higher, on some tiled roofs.

Another issue facing installers and customers is competition for roof space and deciding on which solar technology should have priority. Covering a roof with PV modules only uses 10% to 15% of available solar energy and eliminates the possibility for future solar thermal systems with much

higher solar conversion efficiencies. A client may not be able to install solar panels to heat water, a pool or the building when the roof is already covered with a solar electric technology.

Grid tied PV systems have a high initial cost and are generally sold only with generous incentive programs. A possible solution to the long payback situation is to see whether the "waste" solar heat can be recovered and used to lower heating costs.

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