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When the technology of solar thermal and photovoltaics come together in a single unit, they combine to make better use of the solar spectrum and deliver both hot water and electricity. Eric Hawkins of Thermatricity explains the concept.
The picture at the top of this page shows a development of around 100 houses, which have two to three PV panels fitted to offset the carbon emissions from the gas boilers in the properties. I suggest that what needs to happen next is to increase the number of PV panels on the roof, but as photovoltaic thermal (PVT) collectors.
What is PVT?
PVT systems are designed to convert solar radiation into both thermal and electrical energy. PVT collectors combine photovoltaic solar cells – which convert sunlight into electricity – with a solar thermal collector, which transfers the otherwise unused waste heat from the PV module to a heat transfer fluid. The combination of these two technologies in one panel represents a more efficient solution than an individual PV or a thermal panel.
A house using a PVT system could have a 100 or 150 litre horizontal thermal store installed in the loft as the hot water pre-heat cylinder. The warmed water it produces would be circulated into the vented thermal store and back to the PVT panels to keep the PV panels cool between the spring and autumn months.
The best working temperature for PV panels is below 26ºC, on the basis that PV cells show reduced efficiency as the temperature rises. However, in reality – depending on where the PV panels are installed – PV panels can reach over 90ºC in Australia and up to 70ºC in the UK.
Case in point
Consider a new home using a 250 to 300 litre hot water thermal store, heated by 3 x 3kW immersion heaters. PV cells would offset some of the carbon emissions during the spring/summer months, while the thermal heated water delivered by the pre-heat thermal store would reduce HW heating costs and carbon emissions.
During winter months after the first ground frost the mains water to a property can be as cold as 5ºC. Under these circumstances, boilers have to work with these cold water temperatures, adding costs and increased carbon emissions. Compare this to a HW system fed with pre-heated cold water from 10ºC to 30ºC, thanks to the use of PVT technology.
The assembly of the open vented thermal store in copper or stainless steel provides the store of water to be heated by 3 x 3kW immersion heaters.
Inside the thermal store are 2 x 22mm stainless steel corrugated 316L coils. The two heat exchangers are joined together on the outside by an in-line flow meter, which allows the flow of mains water to be set to what best meets the home’s hot water needs.
The hot water out is via a thermostatic mixer valve. The first 3kW immersion heater is located at the base of the store, wired up to night time electricity rates. The second 3kW immersion heater is located 10in above the bottom one, as the daytime back-up to nighttime cheap rate. This can either be on a timer or switched on manually.
The third immersion heater is a boost for the top part of the store that provides a guarantee of heated water to 80ºC, which services the hot water at mains pressure indirectly through the top coil.
How it works
1. The PV output goes to the grid where the homeowner is paid whatever the rate is for excess. Or a PV booster can be used to dump excess electricity through the second immersion heater, but this will not be enough to support heating.
2. The heated water from behind the PV panels is circulated via a standard central heating pump and solar controller direct through a 100 litre horizontal mounted thermal store in the loft.
3. A cold mains supply of water is connected to the heat exchanger coil inside and back out down to the main thermal store of 250 litres.
4. Every time hot water is drawn from any tap/shower outlet, pre heated cold water will flow from the loft thermal store to the main thermal store below.
5. A second option to PVT is a solar thermal heat pipe evacuated tube collector which would not require a circulating pump or a controller.
How the heating works
On the heating side of the system, it’s the water in the store that is circulated through underfloor heating, warm air flow wall mounted units, or low water content radiators.
1. Cheap rate nighttime electricity is hard wired via a meter to the lower 3kW immersion heater from the time the grid supplies this cheap rate.
2. The immersion heater is set to 80ºC to provide maximum hot water in the store.
3. A circulating pump on a time clock/programmer is set to switch on at 6am, with a central room thermostat set at 20ºC.
4. Once the house has reached its set temperature the pump stops working until more hot water is called again.
5. Depending on the household’s lifestyle – whether a retired couple or a working family – the second 3kW immersion heater comes on (timed or manually) when required.
6. To have a guarantee of hot water for early morning washing and showers, the third immersion heater located in the middle of the store is timed to switch on between 6 and 7am, also set at 80ºC, so there is plenty of hot water flowing out via the heat exchanger coils of 45-50ºC. Once there is a set time for early morning washing, this third heater is switched off, but programmed to switch back on at 5 to 6pm and off at 8pm for evening use.
Not only is this system an affordable solution for many households – for social and council housing– the maintenance is also very low, compared to an unvented cylinder and a heat pump.
A typical cost would be around £3000, plus the heat emitters, which compares very favourably to a PV/heat pump and cylinder system costing £12,000.