PV System with Thermal Heat Recovery

by K Sheinkopf, CADDET US National Team

Introduction

The growing popularity of building-integrated photovoltaic (PV) systems is driving increased use of PV in building designs around the world. While many of the technical problems of this technology have been resolved in the laboratory and in actual rooftop systems, one persistent problem remains: heat build-up behind the cells. As their temperature increases, the performance of PV cells decreases, and waste heat needs to be eliminated for the cells to operate efficiently. Thanks to the support of the US Department of Energy's 'Building Opportunities in the US for Photovoltaics' programme (PV-BONUS), a North Carolina design firm has developed an innovative building-integrated PV system that uses the waste heat from the array to heat water.

 The Applebee's restaurant showing the PV roof section.

System

The roof-integrated system has 32 amorphous PV modules in a sloped roof over a south-facing dining area of an Applebee's restaurant. The system uses 20 kWh of battery storage and an inverter to provide a 1.6 kW-rated system. The batteries store energy collected from sunlight for use during peak electrical times as well as providing an uninterruptible power supply during electrical outages.

What makes this system different from other building-integrated ones is that these same modules that generate electricity also collect thermal energy which is used to heat water. Eight of the rooftop modules are connected to a dc fan that circulates air through a series of passages underneath the 32 modules. About 7% of the total area of the system is clear glass between the PV cells, facing a black-painted high-absorptance metal pan, which actually increases the amount of heat recovered. As the solar radiation increases and the temperature goes up, a fan automatically switches on to circulate this heat away from the PV modules and towards the heat exchanger. In a typical PV system, the air circulating behind the modules is vented away to keep the modules cool. But in this system, the heat flows through a closed-loop air system to the heat exchanger so that it can be used instead of wasted. The big demand for hot water in the restaurant uses all the hot water that is produced.

Economics

There are several aspects of this system that make it so cost-effective. Since it is a building-integrated system, the PV modules replace some of the conventional roof finish, helping to reduce standard roofing costs. The cost of the clay tile typically used on the restaurant chain's roofs is between $86 and $108/m2, with the total conventional roof assembly costing about $162/m2 (where $ is the US dollar). Saving the cost of these standard roofing materials greatly lowered the actual system cost. Brown-coloured PV panels were used to co-ordinate with the restaurant colour scheme so that the aesthetics were maintained, another factor important to the system owners. The PV modules measured 77 cm by 157 cm and cost about $151/m2. The designers also purposely chose these large-sized PV modules to reduce the cost of the aluminum support frames. The selected modules resulted in a glazing system cost of $65/m2, resulting in a total PV system cost of around $215/m2. This is about $54/m2 higher than the cost of a conventional roof for the facility, but the PV system on the roof produces electricity and hot water as well as giving a weather-tight roof finish. A notable feature of the PV system was the installation time; even though this was a new system, workers were able to complete the entire roof and the ductwork in less than three days.

Using this new technology, a typical (all-electric) restaurant will save about $3,000/year in utility costs for electricity and hot water. Savings could vary between $1,000 and $5,000 depending on local energy rates. In addition, each year the system will eliminate 22,680 kg of carbon dioxide emissions that would have been produced by fossil fuels.

Diagram showing detail of the PV roof.

Schematic of the photovoltaic system.

Conclusion

New designs for PV systems will help expand the market for the technology, especially among architects and designers who can incorporate them into their new building plans. Systems like the one described here maximise the potential value of the total solar energy system, providing electricity and hot water while saving enough energy to pay back the system costs in a relatively short time. The unique aspect of this system, the use of the excess heat, actually helped improve the overall performance of the PV system. By taking the heat away, the system reduces the temperature of the panels to below that of an unventilated, insulated assembly, resulting in improved electrical performance. Finally, the environmental savings are also significant, adding to the value of this system.

The system was developed by Innovative Design, an architecture firm experienced in designing solar buildings. Other PV-BONUS team members were Applebee's (a national restaurant chain), the North Carolina Solar Center (whose staff members conducted extensive testing and monitoring of the system), Central Carolina Bank, and Duke Energy.

For more information on this system, contact Michael Nicklas, Innovative Design,

850 West Morgan Street, Raleigh, NC 27603, USA. Tel: +1 919 832 6303. Fax: +1 919 832 3339.

The CADDET Renewable Energy Newsletter is a quarterly magazine published by the CADDET Centre for Renewable Energy at ETSU, UK.

The articles published in the Newsletter reflect the opinions of the authors. They do not necessarily reflect the official view of CADDET.

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