Renewable Domestic Power: Some Thoughts

Heating a Home

Average figures for houses at our lattitude suggest that a 1970's-construction house will need about 13000kWh annually, while a modern house will need about 4000kWh annually. Most of that is in the winter months - so January might need 1000kWh (3000kWh) per month, or 33kWh (100kWh) per day, or an extra 1.4kW (4.2kW) for heating. With a little extra insulation, we might be able to get away with doubling the requirement for the winter months. However, energy for domestic heating is best stored in a thermal store, since these are a tiny fraction of the cost of an electrical store.

To do our calculations for this house, which has been better insulated than it was when it's made, the heating is assumed to require 15000kWh annually (average outside temperature 10oC), of which 2000kWh is needed in each of December, January and February (average outside temperature 5oC.

For comparison, our electrical needs are going to be something around 270kWh/month. Heating is clearly going to be the bulk of our needs.

Sources

Looking at the Electricity Page shows that the times when most heating is required are also the times when the wind is blowing. Windy weather is normally cold, so the best strategy is almost certainly to use a windmill to heat the house. A windmill big enough to heat the house will certainly be able to provide the electrical load without effort.

Since electrical storage is very expensive, the correct way to store this energy is as heat.

The Heat Store

The heat store needs to work between the maximum temperature of liquid water and the minimum temperature for radiators to work. Those temperatures are about 100oC and 40oC respectively. The ideal specification for a heat store is one that will provide heat for a whole winter. But one that lasted us through a five-day calm overcast would probably be sufficient.

A ton of water stores for each 1oC rise a total of 4,200,000kJ, or 1.167kWh. A minimum temperature for domestic hot water might be 40oC and a maximum temperature might be 100oC. The output from the heat store must be mixed with a fraction of cold water to produce safe temperatures. Between these two temperatures, water stores 70kWh per ton.

To prevent the breeding of Legionella and other nasties, the water in the heat store should not be exchanged with the outside world, and should regularly exceed 60oC. Obviously several cubic metres of water at close to boiling point is also a safety hazard should someone fall into it, or should the store suddenly fail and someone be caught in the flood.

Our heat stores need to be insulated: If we use the "five day calm overcast" model, the size of the heat store is now 165kWh. That can be stored in 2.4 tons of water. That size of heat store is pretty tiny: about a cube 1.33m across. In practice we'd probably use a 2x2x2 metre cube.

That small heat store needs to be insulated, of course: if we use the same specification as the electrical store, we want to lose no more than 50% a month, or a total of 2.7kWh per day, or 112W. If the difference between inside and outside the tank is 80oC, and the tank area is 10.667m2, then we need to insulate to 0.131Wm-2K-1. This sort of value might be obtained using 100mm of polyurethane board and a concrete block construction.

Pumping heat into the heat store should be done at the bottom. That's probably best done with circulating water pipes. All the heat input stuff should be as near the floor as possible, to allow heat to rise and be stored in the tank. The power from the wind generator will be dumped into three 3kW immersion heaters.

The heat store should also store hot water generated from the backup generator, and from the kitchen range. This allows backup heating if needed.

Pumping heat out of the heat store should be done with a heat exchanger that rises through the tank. So water to be heated should enter at the bottom and pass up through the tank, keeping the temperature difference as small as possible. This allows the temperature gradient to be maintained, and so allows the maximum heat to be removed from the input water. This is especially useful if thermal solar panels are used.

Each heat exchanger should be fitted with a temper valve, to allow the temperature of the water to be controlled. Setting the temper valves to 40oC for both hot water and underfloor heating should allow safe and efficient operation of both.


This page is some notes on Domestic Power from Renewable Sources, and is written and maintained by Simon. At this stage these pages are constantly under revision. Thoughts and comments are welcome.