Solar hot-water systems
Last updated: July 9, 2009.
Next time you get a really big electricity or gas bill, your
thoughts may turn to solar panels. Wouldn't it be good if you could
catch all the power you need from the Sun? Millions of people already
do get their energy this way, though mostly in the form of heat
rather than electricity. Solar electric panels (also called solar cells
or photovoltaic cells) that convert sunlight to electricity are
still not widely used; solar thermal panels, which use sunlight to
produce hot water, are much more common. Even in relatively cold,
northern climates, solar hot-water systems can chop
significant amounts off your fuel bills. Typical systems generate
anything from 10–90 percent of your hot water and pay for
themselves in about 10–15 years (even sooner if you're
using them for something like a swimming pool). Let's take a closer look at how they
work!
Photo: Making hot water for free (well, once you've paid for the equipment, anyway). This large solar heating system
is on the sun-facing roof of a home in Golden, Colorado.
Photo by Warren Gretz courtesy of US Department of Energy/National Renewable
Energy Laboratory (DOE/NREL).
How to build a solar heating system
Imagine you're an inventor charged with the problem of developing
a system that can heat all the hot water you need in your home.
You've probably noticed that water takes a long time to heat up?
That's because it holds heat energy very well.
We say it has a high specific heat capacity and that's why we use
it to transport heat energy in central heating systems. So can we devise a simple solar
heating system using water alone?
Stand a plastic bottle filled with cold water in a window, in the Sun, and
it'll warm up quite noticeably in a few hours. The trouble is, a
bottle of water isn't going to go very far if you've a house full of
people. How can you make more hot water? The simplest solution would be to
fill lots of bottles with water and stand them in a row on your window-ledge.
Or maybe you could be more cunning. What if you cut the top and
bottom off a plastic bottle and fitted pipes at each end, feeding the
pipes into your home's hot water tank to make a complete water
circuit. Now fit a pump somewhere in that loop so the water
endlessly circulates. What will happen is that the sunlight will
systematically heat all the hot water in your tank (although it'll
never get particularly warm because plastic bottles standing on
window-ledges aren't that brilliant at collecting heat). But, in
theory, you've got a working solar heating system here that's not a
million miles away from the ones people have installed on their
homes. It's very crude, but it works in exactly the same way.
Photo: The pipes that carry hot water to and from a roof-top, solar-thermal collector and into your house.
Tight rubber seals (collars) keep cold rain from entering too! Photo by Warren Gretz courtesy of US Department of Energy/National Renewable
Energy Laboratory (DOE/NREL).
The parts of a solar-thermal hot-water system
In practice, solar heating systems are a little bit more sophisticated than this. These are the main parts:
Collector
This is the technical name for the big black panel that sits on your roof. Smaller homes (or ones
in hotter climates) can get away with much smaller panels than larger
homes (or ones in colder climates); typically collectors vary in size
from about 2–15 square meters (~20–160 square feet). Not surprisingly, collectors work
most efficiently on south-facing roofs that have a direct, unblocked
view of the Sun (with few trees or buildings in the way). Broadly
speaking, there are two types of collectors known as flat-plate and
evacuated tube.
Flat-plate collectors
Flat plates are the simplest collectors: at their most basic,
they're little more than water pipes running through shallow metal
boxes coated with thick black glass. The glass collects and traps the
heat (like a greenhouse), which the water running through the pipes
picks up and transfers to your hot water tank.
Evacuated tubes
These are a bit more sophisticated. They look like thicker
versions of fluorescent strip lights, but work more like vacuum flasks.
Completely empty (hence the name "evacuated"), they collect and
trap the heat from sunlight. This flows to a collecting
device (sometimes called a manifold) at the top (or at one end) through which water
or another fluid flows, carrying the heat to the hot water tank.
Unlike flat-plate collectors, they don't let as much heat escape back
out again, so they're more efficient. However, since
they're a bit more hi-tech and sophisticated, they are sometimes more
expensive.
Photo: Two types of solar-thermal collector: Left: A flat-plate collector. Photo by Alan Ford.
Right: An evacuated-tube collector. Note the grey manifold at the top and the white water pipe flowing through it. Photo by Kent Bullard, US National Park Service. Both photos courtesy of
US Department of Energy/National Renewable Energy Laboratory (DOE/NREL).
Hot water tank
There's no point in collecting heat from your roof if you have
nowhere to store it. With luck, your home already has a
hot-water tank (unless you have a so-called gas "combi" boiler
that makes instant hot water) that can be used to store heat from your collector; it's a kind of "hot water"
battery that you heat up at conveniently economic times (usually at
night) ready for use during the day. If you don't have a hot-water tank,
you'll need to have one fitted. The more people in your household, the
bigger the tank you'll need. A typical tank for a
family home might be about 100–200 liters (30–60 gallons).
Heat exchanger
Typically, solar panels work by transferring heat from the
collector to the tank through a separate circuit and a
heat exchanger.
Heat collected by the panel heats up water (or oil or
another fluid) that flows through a circuit of pipes into a copper
coil inside your hot-water tank. The heat is then passed into the hot
water tank, and the cooled water (or fluid) returns to the collector
to pick up more heat. The water in the collector never actually drains
into your tank: at no point does water that's been on your roof exit
through a faucet!
Pump
Water doesn't flow between the collector and the tank all by
itself: you need a small electric pump to make it circulate.
If you're using ordinary electricity to make the water flow, the energy
consumed by the pump will offset
some of the advantage of using solar-thermal power, reduce the gains
you're making, and lengthen the payback time. Cleverly, some
solar-thermal systems use solar-electric (photovoltaic) pumps
instead, which means they are entirely running on renewable energy.
A good thing about a design like this is that the solar pump is most
active on really sunny days (when most hot water is being produced)
and less active on cold, dull days (when, perhaps, you don't want your
solar panel to be working at all).
Photo: A different and much bigger solar hot-water system. This one uses parabolic mirrors to collect the Sun's energy
and focus it onto water pipes running through their centers. The water is pumped back to the building in the background
(Jefferson County Jail in Golden, Colorado). Photo by Warren Gretz courtesy of US Department of Energy/National Renewable Energy Laboratory (DOE/NREL).
Control system
If it's the middle of winter and your roof is freezing cold, the
last you thing you want is to transfer freezing cold water into your
hot water tank! So there is also generally a control system attached
to a solar-thermal panel with a valve that can switch off the
water circuit in cold weather. A typical control system may incorporate some or all of
the following: a pump, flowmeter, pressure gauge, thermometer
(so you can see how hot the water is), and thermostat (to switch off the pump if
the water gets too hot).
