Sunday, 4 April 2010

Heat - Units

I probably learnt most of my basic thermodynamics from a song by Flanders and Swan called the second law of thermodynamics. This states "Heat won't pass from a cooler to a hotter" and the song has the refrain "You can try it if you like, but you'd far better notter"! They also mention the first law, heat is work and work is heat. I think this was proven by a brewer in Manchester called Joule, who gives his name to the unit of energy required to raise one kilogram of water by one degree.

Joules are tiny units and a more tangible figure for energy is the kilowatt hour. Watt spent some time with steam engines, and his unit measures power. Power is simply the exertion of energy, so if you're exerting one joule every second, that's one Watt. Use a thousand watts for an hour and you have a kilowatt hour. Bringing it home, if you leave a hundred-watt lightbulb on for ten hours, that will use a kilowatt hour. An electric oven uses about a kilowatt, so leaving that on for an hour will use a kilowatt hour.

The sun also gives out about a kilowatt of heat every square metre of earth. We can't use all of this as a bit gets lost in the atmosphere, and the square metre may not be at the same angle as our square metre of window or of solar panel. Even with south facing windows, the sun is always going to hit them at an angle, unless they are tilted inwards. The solar thermal panels are likely to produce 2.3 kWhours in hot water for each square metre every day in the winter. The question of how many we need is tricky. The hot water needs of the house will probably be 12 kWhours per day. This will not change much throughout the year. The heating needs on a very cold day in the winter could be 36 kWhours. Even in the middle of winter, on a sunny day, plenty of heat will get into the house and there will be days when we don't need any heating.

The panels are PVT, which means photovoltaic thermal, so they produce both heat and electricity. They are rated at a little over 30 volts, so we need three to make 100, which is a usable voltage. We probably need some multiple of three panels, although I'm sure it would be possible to put them in parallel and series. Anyway, 6 panels would produce enough hot water for most of the year, but not enough heat for a few cold days in the winter.

We can store some heat, but there's a limit to how much and for how long.

Whatever happens, we need some backup heating system. For example, it may snow for a week, and solar panels will definitely not work if they are covered in snow. As we are connected to the electricity grid, and if we can use off-peak electricity, which should compensate for us adding to the grid at peak summer times, then electricity seems a reasonable backup heat source. We may need to think of an alternative if the economy collapses, but I think we'll be OK for now! Electricity is not that efficient for producing heat, which is not a big problem if we don't use a lot of it.

My car is not so efficient but it averages only about 30 km per week, so probably has the lowest emissions of any in the neighbourhood.

The cheapest heating system is a heating element, like you get in an electric kettle. One of these could be put into the hot water tank that the solar panels supply and switched on at night when the water's too cold.

A more efficient way of using electricity for heat is a heat pump. A heat pump uses slightly more advanced thermodynamics than in the song. Basically it is like an inside-out fridge. A fridge pumps heat from inside the fridge to make it cooler. Somewhere on the outside of the fridge it is hotter. Heat pumps are also used in air conditioning, where they take heat from a cooler room and pump it into the heat outside. In the case of heating, the heat pump takes heat from outside, where it is cold, and brings it inside, where it is hot.

This is in blatant defiance of the second law of thermodynamics, "heat won't pass from a cooler to a hotter". The way they cheat is basically by a compressor. Going back to the first law: heat is work and work is heat, so at a molecular level, heat just means that all the little bits of matter are moving around. If it's hotter, they move around more. If it has less heat, they move around less. This seems to be the opposite of people...

One difficulty in explaining this is the confusion between heat and temperature. For a given object at a given size, adding temperature will increase the heat, and increasing the heat will raise the temperature. However, if you take a gas and compress it to half the volume, the same amount of heat will now be contained in a much smaller size, and the temperature will correspondingly rise. Back in the heat pump, this can then be passed through the warm place you want to get hotter. Next it passes through a valve, then drops in temperature, and can now pass through the cool place that it will be colder than, and so will absorb heat from. And so the cycle goes around again.

Producing heat in this way, or rather sucking in heat from thin air, uses much less energy than making the heat directly from energy. With a kilowatt of electricity, you can bring in heat at up to five kilowatts.

Commercially available systems in Japan are called eco cute. "Kyuto" means water heater, and sounds like "cute". "Eco" is a reprehensible advertising prefix, that I feel refers more to economics than to ecology. The greek root of both words is the same. These water heating systems use off-peak electricity to run heat pumps that suck heat from the air outside. Although it makes economic sense to do this, as electricity is cheap at night when nobody is using it, it makes less sense from an energy point of view. Heat pumps run less efficiently the lower the temperature gets, although they will still be able to suck some heat out of the air when it gets down to ten or fifteen below zero. In our case, the time we're going to need the extra heat is on cold days in winter, when the heat pump will be at its least efficient. The coldest days are sunny, so actually it will not quite be the coldest days, but the days we need extra heat will certainly not be warm.

It may still be that as a product eco cute turns out cheaper than patching together our own system for delivering hot water and underfloor heating, although we need to find a way of feeding the water from the solar panels into it as they are usually supplied from the water main. This system would cycle water from the panels through one tank, than take hot water from this tank into the eco cute. The eco cute would then use its mass produced control system to send this hot water where it is needed, and to switch on the heat pump when it's getting too cold. It might be upset that it is not using its heat pump very often, although that's tough!

The other system would simply use one tank, heated by the solar panels, backed up when there is not enough heat by a heating element, feeding hot water to the house, and heating either a pipe for underfloor heating or heating the air that is sent under the floor.

The control systems we'd need would be:
1. when to pump water from the solar panels, which we're on our own for with or without an eco cute
2. when to add extra heat, which we wouldn't want to do other than at off peak
3. when to send heat into the underfloor heating.
4. I'm becoming less and less convinced we need this, but if we are going to reheat the bath water, then we need it decide when to do that.