Throughout the course of human civilization we've enjoyed what we believe to be effectively
unlimited energy and resources in our global playground enabling us to do more or less
whatever we wanted wherever and whenever it suited us without any concern for the consequences.
And when there were only a few hundred million of us scattered around the earth
that philosophy was probably quite valid. But there's nearly 8 billion of us now, and we'll
be more than 10 billion by mid-century, so the old mindset of 'nature's infinite bounty' doesn't hold
true anymore and we've reached a point where our earth systems are really creaking at the seams
to cope with our rapacious demands. Most rationally minded people have at least recognized
that the major causes of our current predicament are over consumption and of course the burning of
fossil fuels. Our policy makers all know very well that we need to very rapidly move to low
carbon sources of power like wind and solar in what will become a much more electrified future,
but that paradigm shift will ask an awful lot of our electricity grids in the coming years.
By 2030 most people who own a car will be driving models powered by batteries not hydrocarbons.
Millions of homes around the world will be running electric heat pumps instead of gas boilers.
Significant additional strain will be put on our grids by more and more data centres to feed our
insatiable addiction to social media and digital information, and all of that will be exacerbated
by things like cryptocurrency mining facilities and the rapid increase in air conditioning
systems. So I think it's probably fair to say we no longer have the luxury of being profligate in
the way we use the energy and resources available to us on our little blue galactic spaceship.
In fact what we need to do is find really smart ways of optimizing the energy we generate so that
virtually none of it is wasted and therefore the amount we need to generate in the first place is
minimized. Solar panels are a great example. The best technology available on the market today
has a sunlight to electricity conversion factor of about 18 to 20 percent. That means more than
80 percent of the energy hitting the panel is simply wasted as heat. And actually,
because of physics stuff that we won't go into here, as the heat builds up on the panel
it reduces the panel's ability to produce free electrons to make electricity. So ironically on
a nice hot sunny day in summer a solar panel rated at 20% efficiency may only be achieving about 12%.
So here's an idea straight out of the Edward de Bono school of lateral thinking...
why not find a way of capturing that excess heat, drawing it away from the solar panel,
and doing something useful with it? That way you kill two birds with one stone - you get
more efficient solar panels and you use less energy producing heat somewhere else
because you can just use the energy the sun has already provided. Sounds a bit obvious when you
say it out loud doesn't it, but it's only very recently that people have been developing it into
a marketable proposition. It's called photovoltaic thermal or PVT, and it could increase the amount
of energy your solar panels can harvest by as much as three times. So let's take a look.
Hello and welcome to Just Have a Think. Now you might be thinking 'hang on Dave,
surely rooftop solar heating has been around for decades?' And you would of course be quite
correct. Jimmy Carter had solar heating panels installed on the roof of the White House while
he was president in the late 1970s, before Ronald Reagan had them removed during his administration.
Rooftop solar hot water heaters can achieve sunlight to water heat conversion efficiencies
as high as 50% or more, but they have very low capacity factors. So what does that mean? Well,
a typical system is designed to meet water heating demand even in the depths of winter but in those
colder months it can take most of a day to meet the heat demand of the household water system.
On a sunny summer day the water can get piping hot within an hour or so,
and that means all the solar energy for the entire rest of the day is completely wasted. By contrast
solar photovoltaic panels generate electricity that can either be used immediately or stored in
batteries for later use by the householder. But unless you've got a roof the size of a
small warehouse you're only likely to have one or the other installed for your domestic needs.
And because in most parts of the western world natural gas for home heating has, at least until
recently, been so cheap and gas boilers have become so efficient, solar thermal panels have
largely lost out to solar photovoltaic panels in a battle for that limited rooftop real estate.
But what about that Solar PV panel inefficiency factor?
According to this 2017 analysis paper by the Swiss federal funding programme Energie Schweitz,
"approximately 10% of the solar irradiation on a crystalline photovoltaic cell is reflected and
cannot be utilized. Around 17% of the remaining 90% of the irradiation that is absorbed by the
cell can be converted into electricity and 73% is converted into thermal energy.
In a photovoltaic module the thermal output remains unused. It raises the temperature of the
cell and can thus have a negative effect on the electrical efficiency of the module". And here's
that again in layman's terms... a typical solar panel is rated for optimal efficiency at a cell
temperature of about 25 degrees Celsius. That's not ambient surrounding air temperature, that's
the solar cell temperature. For every 10 degrees Celsius the solar cell heats up above that level,
the panel loses something like 5 percent of its rated performance.
So a panel at 75 degrees Celsius would have lost a quarter of its ability to generate electricity,
and on a sunny day in somewhere like Australia, which by the way has the largest proportion of
solar PV panels per household in the world, it's not uncommon for panel temperatures to
hit 100 degrees Celsius. It's almost the definition of irony isn't it really - a
device that relies on the sun but actually gets less efficient as the sun heats it up.
And the really harmful enemy of solar panels is sustained heat. According to Professor
Martin Green of New South Wales University, who's affectionately known as the godfather of solar PV,
a decrease of 10 degrees Celsius in operating temperature could double the lifespan of solar
panels and boost their performance every day. Now you could stand on your roof with a hose
pipe in your hand spraying all your panels with water during the hottest part of the day.
That would certainly cool the panels down and immediately improve their efficiency.
But you'd probably get bored and eventually you'd get heat stroke and fall off the roof. Plus
you'd be using a bunch of water, which is another precious resource you don't really want to waste.
And natural gas isn't quite as cheap as it once was is it? Over here in Europe,
where bad things are happening, our home heating bills are about to go off the scale. Plus policy
makers in most countries are now looking for ways to urgently reduce the carbon dioxide emissions
from their national energy sector, not to mention reduce their reliance on fossil fuel supplies from
less than reputable sources. So if there was a safe and reliable way to cool down solar PV panels
to optimize their electrical performance and divert that recovered heat to do some useful work
then you'd be on to a winner right? And that's where PVT technology comes in. It's actually
been in development for a few years. Early designs attempted to combine the design of a solar thermal
panel with solar photovoltaic technology by essentially adding liquids in an energy reservoir
heat exchanger box bolted to the bottom of the solar PV panel. The liquid was plumbed up to the
box from the house and a pump was used to control the flow and outlet temperature. But these early
designs came with a few drawbacks. Firstly, the heat exchanger typically had an inlet at one end
and an outlet at the other. That meant the cells closest to the inlet were always cooler than the
cells closest to the outlet. The difference could be quite significant across the panel, and because
of the interdependent way that cells work on a solar panel that temperature variation meant the
performance of the whole panel was only really as good as the hottest cell. And the liquid needed to
be in constant contact with the back of the panel to allow the heat to be dissipated away. Industry
research showed that even with a small air gap the heat transfer dropped exponentially with gap size.
And in colder climates an antifreeze like glycol had to be used to prevent the heat exchange liquid
from freezing. Leakage from joints was also an issue, and you don't really want liquid escaping
into a confined space with high voltage DC current flowing through it. Add to that the complexity of
all that plumbing during installation, and the extra difficulty of repairing or removing the
plumbed in panels, and you've got yourself a bit of a cumbersome solution that could be a difficult
sell to the average punter. The alternative is a PVT system using air or gas like this one from
an Australian startup company called Sunovate. In a recent web chat that I had with Sunovate's
co-founder and technical director Glenn Ryan, he explained how the system works. An airtight
cassette is created using the same inexpensive stamping machines that make car body panels.
The air box is designed to be simple enough that it can either be factory fitted to the underside
of the solar panel or retrofitted to existing panels on a rooftop in such a way that it doesn't
affect the warranty of the existing panels. At each end of the box is a fan that pushes ambient
air in, which then gets heated by the excess panel heat and sent back out of an exit point.
So you're constantly removing heat energy from the underside of the solar panel which means its
electrical generation capacity is being improved and you're harvesting the heat energy to do some
useful work. Sunovate's research showed that on a typical 25 degree Celsius day you can easily
get 40 degrees Celsius of heat energy from the cassette system, which effectively increases the
amount of solar energy being utilized by the panel from 17% right up to about 50% and increasing the
operational life span of the panel from about 20 years to something more like 50 years.
And of course once you've captured the heat energy there's a whole bunch of options for what
you can do with it. It can be fed directly into a ducted system to provide direct space heating
inside the house, or it can go through a heat exchanger to supplement your home's hot water
system. There's even a company called Stiebel in Germany that makes an interior air source
heat pump which could take the excess heat energy directly from the Sunovate system
and use it to provide all of the homes heating and hot water, even on colder days. In hotter regions
like Australia and the southern states of America, where your house may not need any heating,
these systems could dump heat somewhere else like into the pool in the garden,
which I'm told are a popular choice in those parts of the world. If I had one in my garden
here in England it'd probably be more useful as an ice rink to be honest, but the point is you're
still removing heat from a PV system to allow it to operate far more efficiently and sending that
heat into something that would otherwise be using electricity to keep it warm. And as an added bonus
the exact same cassette system can harvest cool air at night time when the surface of
a solar panel is typically about eight degrees Celsius cooler than the surrounding ambient air.
That air can then be circulated around the home to provide a more comfortable night time temperature
in those hotter countries. But these PVT systems also have a great potential in larger
applications. In a commercial setting they could be used for all sorts of services like wood drying
or supplemental heat for industrial processes. And perhaps one of the most promising opportunities
is in district heating systems. Sunovate's plan is to integrate their solar PV heat recovery system
with glass house type structures to create multi-megawatt installations.
District heat can supply residential homes and public buildings as well as commercial greenhouses
and industrial processes. They can also be connected to large seasonal storage facilities
where heat energy can be squirreled away for winter and high demand periods. In Denmark for
example they've got systems that collect heat during the summer and dump it into huge pit
storage systems with insulating foam toppers. And in the Netherlands there are facilities that force
hot air into subterranean aquifers for long-term storage. All of these existing systems could be
greatly enhanced using harvested energy from PVT technology. It's certainly an ambitious goal,
but if it can be achieved then we'd have something that would be making a really tangible difference
to our global decarbonisation challenge. Sunovate themselves have a couple of big scaling steps to
go through before they can provide commercial scale data for their levelized cost of heat
production, but in our web chat Glenn Ryan told me he's confident that once they reach mass
production levels they'll be at least competitive with natural gas with an applied carbon price.
My YouTube buddy Rosie Barnes visited Sunovate's prototype site out in Perth, Australia recently
and she's produced a fascinating video over at the Engineering with Rosie channel looking at how
solar thermal is likely to fit in with the overall renewable energy matrix. And I'll leave a link
to that one in the description section below. You may well have direct knowledge of these systems or
perhaps you're already working in the industry and you can share a few nuggets of insight. If you do,
or if you just have views on the subject one way or another, then jump down to the comments section
below and leave your thoughts there. That's it for this week though. A massive thank you, as always,
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