Geothermal, including heat pumps, both ground source and air source

Before I begin here, I need to clarify some terms.  If, in doing so, I am teaching what you already know, then please skip this section.

First, we need to clearly understand the difference between heat and temperature.

Take a needle and hold it in a candle flame until it glows red.  You wouldn’t want to hold that glowing tip: it will have a very high temperature (about 1400°C) and it would hurt.

But if you had a cup of cold water with a thermometer in it, and you plunged your glowing hot needle into the cup, you’d get a hiss, a tiny amount of steam, the needle would go cold … but you’d hardly see any rise in the temperature of the water in the cup.  The needle was at a high temperature, but, because of its tiny size, didn’t contain much heat.

You could say that if you squeeze heat into a tiny space, you get a high temperature, but the same amount of heat in a large space only gives you a tiny temperature increase.  You will know this to be true from experience.  Go and sit in a very small, cold room with a fan heater going, and it soon gets toasty and warm.  But that same fan heater in a barn can run forever without it seeming to get anything other than cold!

So heat and temperature are different things.  Temperature is measured in °C (or °F) and heat is measured in calories or Calories or joules.  A Calorie is 1000 x a calorie (and it’s the Big-C version you’re worried about if you’re dieting) and a calorie is a bit over 4 joules, and if you want to know when you should use which, check out this webpage (it’s only really of interest to scientists!)

OK, with that settled, let’s get going.

Actual Geothermal

Hot springs in California
We all know it is jolly hot in the middle of the earth–we can see that in places where that heat spills out, as in volcanoes and hot springs.  But even in non-volcanic regions, for every 1 km you drill down into the earth, the temperature rises by 26°C.  In Southampton (one of the few cities in the UK to have its own geothermal energy system) they drilled a hole over a mile deep (1.8km) to where the temperature is 76°C, and they have saved huge amounts of carbon by using that to heat places like the Civic Centre, Southampton hospital, etc.  If you’re into such things, there’s an excellent article on geothermal energy in Wikipedia, including details of Southampton’s scheme.

But we’re not going to be drilling mile-deep holes in the earth at Foldehampton!  So we’re not actually going to be using actual geothermal. Hardly anybody does.

But if you just dig down about 6′ or so (2 metres … or anything down to about 10-15m) you will find that the temperature of the ground is stable all year round, at about 10-15°C.  This isn’t geothermal energy; it’s heat from the sun that works its way down, and is stored in the earth.  We can access that heat using “ground-sourced heat pumps” (although you will find some people referring to this as geothermal energy).

Heat Pumps

If you go outside on a wintery day, when there’s snow on the ground, it’s difficult to appreciate that there’s a lot of heat out there.  That’s because you’re confusing heat and temperature.  If there weren’t a lot of heat out there, the temperature would be about -273°C and even the air would be frozen solid and you’d die in an instant.

You can get hold of that heat and make use of it using something called a heat pump, which is just a device for moving heat from one place to another.  We all encounter heat pumps all the time: a fridge or a freezer is just a box with a heat pump that pumps heat from the inside to the outside (put your hand round behind your fridge or your freezer and you’ll discover it’s quite hot).  An air-conditioner is just a device that pumps heat from the inside of your house to the outside.

Now, I hope that it makes sense that a heat pump has an easier job moving heat from a place where there is more heat than from one where there is less.  Certainly in winter, although there is heat in the air, whatever its temperature, there is more heat a few metres down in the ground.

Domestic heat pumps are usually either ground source or air source.  

Ground source are more efficient (it’s warmer down there)–but you do have to dig a biggish hole to bury your pipes.  The deeper you go, the more heat is available.  There are two good animations on this YouGen webpage that explain it well.

And although ground source is more efficient, they are quite expensive on a  house-by-house basis, and quite often difficult to retro-fit without digging big holes in your garden.

At Foldehampton we won’t by and large, need to worry about heating our houses, because they are built to PassivHaus standards, but they will still use energy for appliances, lighting, hot water, and cooking.  The houses will all be built and oriented to make best advantage of natural daylight, and will be fitted with the most energy-efficient LED lighting we can find.  As for cooking: our houses can be fitted with the energy-efficient induction hobs, which not only use less energy, but are inherently safer (the cooking surface doesn’t get hot, just the pans), are easier to clean, and are as responsive as gas when you want to change the heat.  If you are new to the idea of induction cooking, there’s an excellent and comprehensive Wikipedia article here.

Which just leaves hot water.

One of Washing Machinesthe keys to reducing the cost of living at Foldehampton is to share resources whenever possible, and we’re going to suggest that, instead of each house having a washing machine, that for every (say) ten houses or so there is a laundry facility, equipped with the most energy-efficient washing machines.

These buildings will also be the source of hot water for their (say) ten “client” dwellings.

Each laundry building will have a deep ground-source heat pump, will have its own solar panels, and a hot-water recovery system linked to each client house.  Waste water heat recovery systems are generally thought to be too expensive for domestic use, but the economic case makes considerably more sense when the cost is shared between a group of dwellings, and when multiple systems are being bought for the whole village.

Cold water is piped to the laundry centre and stored in a tank.  The solar-powered ground-source heat pump heats the water, from whence it is made available to the washing machines in the laundry centre, and by way of highly-insulated pipes to the laundry centre’s client houses.

Of course each of these laundry centres will have south-facing “conservatories” where washing can be hung to dry without the need to use electricity, although each may have a tumble dryer for “emergency” use!

Grey water (bath, shower, dishwasher) water is piped back, via insulated pipes, to the laundry centre, where another heat pump recovers the heat (pre-warming the incoming cold water), and then passes the grey water to a biofiltration system so that it can be used for irrigation.

These systems will produce hot water at each house that is probably hot enough for, say, hand washing, but maybe not quite hot enough for a shower.  However, the extra few degrees of temperature boost necessary will be easy to supply from the available solar electricity.

The integration of these various systems should mean the very lowest possible energy bills for each house, possibly as low as £100 per annum for all domestic energy requirements.