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More Info (Ts & Cs)

Energy and the engine

The average diesel engine is about 30% efficient. This means that about 30% of the calorific value of the fuel is converted into rotational energy.
The remaining 70% of the calorific value of the fuel is given off as heat. This heat comes out from the coolant system and the exhaust gases. The exhaust carries more heat than the coolant, so for simplicity we will assume that the 70% is divided into 40% exhaust and 30% coolant proportions.
This means that a 3KW Genset will provide 3KWs of electricity on full load, plus 3KWs of heat coming from the coolant water, and 4KWs in hot gases from the exhaust.

Harvesting the waste heat

Harvesting the heat from the coolant water is relatively easy. There is a flow and return pipe from the engine which can, in theory, be fed directly to the flow and return connections of your house hot water tank.

Harvesting the exhaust gas heat is much more difficult. The exhaust gases need to go through some kind of gas/water (or possibly a gas/air) heat exchanger, which is problematic when you realise that the gases can easily be as hot as 400^oC. Care must be taken to build a system that will never allow steam flashes etc.
More challenges become evident when you look at the chemical composition of the exhaust gases:- There is a large proportion of water (steam), some HydroCarbons, Carbon Dioxide, Carbon Monoxide, Gaseous oxides of Nitrogen, and if you burn fossil diesel, you'll also get some gaseous oxides of Sulphur.
These chemicals are not an immediate problem for the heat exchanger, that is, until you cool the gases to the point where the steam condenses (which is a frequent scenario - when the engine is warming up). Once water is around, the gases start to dissolve forming:- carbolic, carbonic, Nitric, Nitrous and Sulphuric acids. These acids love to eat heat exchangers that aren't made from something that can resist them (e.g. stainless steel).
Reducing the temperature of the exhaust gases also encourages the precipitation of soot, which means a well-design exhaust heat exchanger will be made from stainless steel and will be easy to dismantle and clean.
Reducing the temperature of exhaust gases in a heat exchanger beyond the point of condensation is however, very beneficial from a heat recovery point of view because it recovers the latent heat of evaporation which is a significant proportion of heat.

Electricity and the home

The average UK home consumes around 12KWhrs of electricity in an average 24hr period, which is the same as saying your average electricity consumption is 500W per hour. This means that theoretically you could be very happy with a 500W genset running 24hrs a day, 365 days a year. However, nobody makes a 500W genset that is continuous duty, but that is not the biggest problem.
The average consumption hides the fact that peak consumption can be as high as 10KW (or higher in some homes) - evening time when the iron, kettle, tumble dryer, washing machine, electric shower etc. are on.

So why not just buy a 10KW genset?
The problem with having a 10KW genset in this scenario is that most of the time it will be grossly under-loaded. It will be running cooler than it should be, which most engines don't appreciate as they get carbon deposits in the combustion chamber and around the valves and can even get glazing on the hot surfaces which all work to reduce the life of the engine. It will also be running very inefficiently - consuming more energy than necessary just to move the larger reciprocating and other moving engine parts.

There are several solutions to this problem of peaky consumption. We suggest the following:-

Run your generator when your load is moderate to high, and switch to the grid when it is low.

Run your genset for a proportion of the day (a proportion of a 24 hour period), at heavy load. This will generate more electricity than is needed at the time of generation, so the excess can be diverted in to deep cycle batteries where it can be stored until another part of the day when an inverter will turn it back into usable AC power.

Buy a sophisticated inverter charger that is capable of pulling power from both the generator and battery bank at the same time.

One thing that will help regardless of solution chosen, is to take control of your consumption and avoid having everything switched on at the same time.

The Inverter solution with batteries allows you to trade-off genset maximum output with number of hours running for the genset. For example, (assuming a daily consumption of 12KWhrs) a 500W genset running all the time, could be replaced with a 12KW genset running for one hour (using over-simplified arithmetic to communicate the concept).

Heat and the Home

Running a large genset for a short amount of time is fine in theory, but we are talking about CHP units, not (just) gensets. The 12KW genset will produce 18KWhrs of hot water in that one hour period. How do you go about capturing 18KWhrs of heat and putting it in your home in a useable form? It's not easy, and will be very expensive as you would need a large heat storage system.
A more practical approach would be to run the generator only when you need heat. This normally corresponds to the time when we have our peak electricity demand. A generator capable of supplying power for a moderated peak demand (some discipline is used to ensure all electrical consumption devices are not switched on at the same time) would also provide all your heating needs and would be switched off when electrical demand drops.

Recent tests using the Volvox MultiSpeed Generator and the Volvox Exhaust Gas Heat Exchanger in series with the cylinder block coolant water produced 8.5kWs of heat whilst generating 3.2 kWs of electricity. The test was run using rapeseed oil, for one hour once the heat values had stabilsed. The fuel consumption was just less than 1.5 litres of fuel. Given that there are 9.4 kWhrs in each litre of rapeseed oil, the whole CHP system works out to be around 85% efficient.
[N.B. Further efficiencies could be gained by adding insulation on the pipe work].

Home CHP is still in its infancy in the UK. To help promote this wonderful, environmentally friendly, self-sufficiency philosophy, join the Home CHP discussion group.

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