What Exactly is PVT?
It stands for ‘photovoltaic thermal’. It’s basically a hybrid solar panel consisting of photovoltaic (electrical), or PV, and solar thermal (heating) functionalities that will contribute towards a household’s requirements for electricity and hot water.
PVT is a technology that yields electricity and hot water from one solar panel and it’s making news in the renewables world.
How Does PVT Work?
PVT is a way of cooling PV panels and collecting the heat, via water (or another fluid such as glycol) and air. In the UK we have water-based systems, as heating systems and technology tend to be water-based.
A heat exchanger (typically a series of pipes in an insulated box) is fixed to the back of a PV panel. As the panel is heated by the sun, fluid is circulated through the heat exchanger to remove the heat and keep the PV panel at its optimum operating temperature — not higher than 25°C. The heat collected can then be transferred to a thermal store.
What Does PVT Look Like?
It was originally developed in the United States as a means of improving the efficiency of PV panels by cooling them. So it looks like a PV panel with pipework around the edges.
This has two effects:
- It is difficult to mount on the roof (but not impossible) in place of roof tiles, as some PV and solar thermal panels can be. Instead, it is usually mounted on top of the roof with a clear gap between the PVT panels and the roof tiles.
- The exposed pipework around the edges gives it a somewhat ‘industrial’ look, which would not please every homeowner.
Manufacturers of PVT Systems
There are said to be less than 500 systems installed in the UK, compared to the tens of thousands of PV systems.
Costs of a PVT System
Cost is more than a PV system (expect to pay around 1.5 to 2 times the cost of an equivalent rated solar PV system) but a little less than separate PV and thermal systems of equivalent capacity. A PVT system will produce around 40% more energy than equivalent separate systems. If the increased production is taken into account, the cost per kWh is significantly lower.
Are There any Issues?
A PVT system will produce about three times as much heat energy as it does electrical energy. Heat tends to be in the 50°C to 60°C range, rather than the 70°C-plus that a standard solar thermal system will produce. This means that the heat produced is suitable for space heating.
As it does not qualify for domestic RHI anyway, there is no penalty for using it for that, but for domestic hot water a separate heat source (to raise it to over 65°C for disinfection purposes) will be needed.
Bear in mind that 50% of that heat will be produced in the three summer months, and only about 10% in the three winter months.
Herein lies the problem. It makes little sense to install a system that produces too much energy, so PVT systems are generally sized to meet the household summer heat demand, which makes them too small for winter. This is a dilemma that is yet to be properly resolved — unless you have a swimming pool.
Homeowners installing a PVT system used to receive financial incentives from the government. However, around 2010, the government decided that PVT would not qualify for either the Feed-in Tariff (FiTs) or the Renewable Heat Incentive (RHI).
Solar PVT Project
This project, designed by Hawkes Architecture, uses a roof-mounted 6kW PVT system to harness solar energy as electricity and heat. The heat is extracted and stored beneath the Paragraph 79* dwelling in an inter-seasonal heat store that heats the house during the winter. The electricity is used on site and stored in a battery to be used during peak hours in the evening.
“This stockpiled free energy allows the heat pump to reach a coefficient of performance of over 6.8 (6.8 times more heat out than electricity in),” says Andy Perkins of EnergyZone, the company that installed the PVT panels on the project. “The integrated system harnesses four times more energy from the PVT panels compared with standard PV panels alone.”
Tim’s Analysis: is Solar PVT a Great Idea?
If you are thinking of PVT for the FiTs and RHI income, then find something else to spend your money on. PVT is for people who want to invest in minimising long-term running costs by producing as much energy as possible on site, at the lowest possible unit cost.
What this needs, above all, is a specialist installer. It is a technology with acres of potential but it is not simple technology. In fact it is technology that is, to some extent, still in development, so it needs an installer with that appropriate expertise.