PVT – a technology that yields electricity and hot water from one solar panel – is making news in the renewables world. Tim Pullen investigates.
What Exactly is PVT?
It stands for ‘photovoltaic thermal’. It’s basically a hybrid solar panel consisting of photovoltaic (electrical), or PV, and thermal (heating) functionalities – usually separate – which will contribute towards a house’s electricity demands while heating hot water.
How Does it Work?
PVT?is essentially a photovoltaic collector that produces heat as a byproduct. The panel absorbs photons (electromagnetic radiation) from the sun, and an inverter changes this direct current (DC) into an alternating current (AC), suitable for use in the home. The process naturally generates heat, which is transferred via an aluminium heat exchanger – located on the back of the collector – to a closed circuit through which runs an antifreeze heat transfer fluid (a mix of water and glycol); the fluid takes the heat to the hot water cylinder. When set up correctly, this process actually aids the functionality of the PV module, as it causes the heat in the cells to dissipate — and PV cells are more efficient when they are cooler.
So far there are only two contenders in the UK market — New Form Energy, distributor of the German Solimpeks system, and Anaf Solar, an Italian manufacturer. New Form Energy has been operating for a few years and now has a good number of installations up and running. Anaf Solar has a solid reputation across Europe as a PV manufacturer and has been manufacturing and installing PVT since 2007. Both systems operate in largely the same way, with a heat exchanger fitted to the back of a PV panel.
Are there any Issues?
There is one potential problem in that heat output can be three times the electricity output. If we assume a 4kWp system, a standard PV array of that size would produce 3,000 to 3,200kWh of electricity each year. A PVT system will produce over 3,500kWh of electricity and up to 10,500kWh of hot water each year. That sounds fine, as an average house with four people in it will need about 4,000kWh for hot water and 8,000 to 10,000kWh for space heating. But, and it is quite a big but, PVT will produce around 50% of that hot water in the three summer months — some 5,250kWh when we actually need only 1,000kWh. So what do we do with the rest?
New Form Energy has a panel with a 1:1 ratio, matching heat production to electricity production, which solves the problem but looks like a bit of a regressive step. It also has heat storage technology so that summer heat can be stored for winter use. Anaf Solar has linked its system to a heat pump to amplify the winter hot water production to useful levels. Useful enough to supply the heating and hot water to a well-insulated property.
PVT is accredited under the MCS scheme, hence a single panel will qualify for both the Feed-in Tariff (FIT) scheme and the Renewable Heat Incentive (RHI) scheme. Whilst the FIT is fully operational, the RHI scheme has been consistently put back by the government, with the current estimated time for introduction being 2014.
Architect and self builder Richard Hawkes installed one of the first PVT systems in the UK on his zero-carbon PassivHaus in Kent (main image above). Originally planning for an evacuated tube solar thermal system, New Form Energy asked Richard to install a prototype PVT system. The process has been a learning exercise for the company and Richard, but he is delighted with its performance so far: “Thermally, the system is brilliant, but we did specify the wrong size inverters for the PV element, reducing its capabilities by about 40%. The panels also aren’t cooling as they should, so we are working to equalise the flow of the heat transfer fluid to extract more heat from the thermal element.” These two improvements should dramatically improve efficiency.
One of the more unique aspects of this system is the use of salt-based phase change material balls within the 400lt cylinder, around which water circulates.?Richard claims that phase change materials are up to seven times more efficient than water at storing heat, retaining the heat for long periods of time — for use on days when the sun isn’t as bright.
The system size is 27m2 2.95kWp with annual outputs of electrical: 3,408kWh (set to increase); and thermal: 12,064kWh. Last year, which included one of the coldest winters on record, Richard paid a tiny £25 in utility bills. hawkesarchitecture.co.uk
Tim’s Analysis: A Great Idea?
By the end of 2009, the UK had installed a total of 400,000m2 of PV panels. That is just 20% of the PV capacity that Germany installed in 2008 alone. PVT, with the extra financial returns it brings, may be the kick-start that solar energy needs.
The reality is that we need renewable energy now and solar energy is the one form that most of us can aspire to — hydro is available to very few, wind largely excludes the urban dweller and too much biomass will bring back the smog.
PVT works and works well. It has the potential to provide all the hot water and space heating energy of a well-designed house, with zero running cost and zero CO2 emissions. It is, with hindsight, an obvious and necessary development of PV and solar thermal — but probably not the last one we will see. The technology has flaws and there is room for improvement, but it is the best we have so far. And it would not have happened if people had not invested in the earlier, less efficient, products.
A short solar history
Solar thermal and photovoltaics are both fairly old technologies. The first known solar panel designs date back to Leonardo da Vinci, who, in the late 15th century, conceived an early use of solar power by employing concave mirrors to heat water. However, the idea of bringing electricity and hot water together in a single panel has only recently occurred — and like a lot of inventions, it occurred by accident.
Using solar energy to produce hot water has been around as a commercial product since the 1960s, but really started to come into its own in the late 1990s, driven by rising fuel prices. Similarly, it was the NASA space programme that provided the funds to develop PV technology into a commercial product. PV was fairly inefficient, so the industry researched improvements, thus discovering that keeping panels below 25°C kept them at optimum performance levels and improved electricity production by as much as 30% — this was a huge step forward. In addition, the volume of hot water produced was useful in its own right, and so the idea of a hybrid panel was born.