My house is not connected to the electricity grid (too far away from any existing power lines). Our power comes from (a) a 3.9kWp array of PV panels and (b) a 10kVA diesel generator.

From this we run a normal house, 230V AC, with a typical "family" level of consumption.

For 6 months of the year the PV panels produce pretty much all the electricity we need. In the winter the panels cannot possibly produce enough electricity and this is where the diesel generator comes in. The diesel generator cuts in automatically when needed and this all works pretty well.

The generator produces heat as a by-product and at the moment all this heat is "thrown away". Since we have to also heat the house, it occurs to me that it would be a very good idea to capture the waste heat from the generator and use this in the house.

We are in the process of installing a 1400 litre thermal store, so my idea is that the cooling water from the generator could form one of the inputs to this.

The diesel generator itself is just like a car engine and has the same kind of cooling system: - radiator, water pump, fan, thermostat etc

Has anyone had experience of retro-fitting a heat recovery system to an existing generator? Is this feasible and/or worthwhile? What about lengths of pipe runs (the generator is currently about 70 metres from the house, but could perhaps be moved nearer)?

The OU featured a Fiat engine as part of a domestic CHP system when they dealt with the subject as part of their Technology course back in the 80s, recovering heat from both the cooling system and the exhaust, but it's proved surprisingly difficult to source info now.

The Navitron forum had a thread that was titled for CHP, but for electricity generation only, no mention of capturing the waste heat... http://www.navitron.org.uk/forum/index.php?topic=922.0

Some interesting stuff here, including a system schematic... http://www.epa.gov/chp/documents/catalog_chptech_reciprocating_engines.pdf

What are your comments on the simple system described in the second link, Nick?

Posted By: JoinerThe Navitron forum had a thread that was titled for CHP, but for electricity generation only, no mention of capturing the waste heat

There are lots of threads on the subject. Search for 'frotter' as a start :)

Posted By: barneyOnce you remove that fan then the set is subject to very rapid overheating

But you are replacing it with a different fluid (water) that has a much higher SHC (4.2 as opposed to 1). So you only have to pump a quarter of the mass through a heat exchanger (I use the term loosely).
But I would leave the rad in place and just bypass it when heating water. I think a 10kVA (is that 43 Amps@230V) generator is going to over produce heat anyway (possibly up to 12-15 kWh).

Not actually checked water temperature in a static engine, though I thought cars used pressurised systems to limit evaporation and allow for use at high altitude.

A reasonably good diesel engine will be around 30% to 35% thermally efficient, so will waste around 65% to 70% of the potential heat that has come from burning fuel. If you could recover all of the waste heat from a diesel gen set that is running at 10kW, then you would get somewhere around 23kW, probably more than you could comfortably deal with. Much of the waste heat is carried away by the exhaust though, so the cooling system usually only has to get rid of around half (maybe less) of the wasted heat, maybe 10 to 12kW when the gen set is running at full load. This is a fair bit of heat to pump into a thermal store, especially if you run the gen set for long periods of time.

If you didn't want to mess with the gen set cooling system you could use an exhaust heat recovery system only. Not only would this probably give you a more manageable heat input but it may also make the exhaust a fair bit quieter. Some of the Lister (and Indian clone Lister) gen set enthusiasts have done this with buried drum silencers/heat exchangers, but a friend has made a really simple exhaust heat exchanger simply by wrapping 10mm microbore copper pipe around a few feet of the exhaust pipe. He finds he can run a conventional central heating system with a couple of radiators like this quite well, so I think it would work OK for charging your thermal store. If you arranged it as a drain-back system for over-heat protection, similar to the sort of system you might use on a solar hot water system, then you could allow for cutting the heat off when the thermal store reached max temperature with the gen set still running.

The only thing to watch if using exhaust heat recovery is the possibility of exhaust system corrosion from excess condensate. This can be reduced or eliminated by ensuring there is a slight fall to the exhaust system so that any condensate can freely drain away through the exhaust outlet.

The other thing that helps is to lag any parts of the exhaust system that aren't part of the heat exchanger, to maintain the exhaust gas temperature at as high a level as possible.

Posted By: JoinerTried djh, but got...http://www.navitron.org.uk/search.php?search_in=all&search_text=frotter&x=17&y=11" >http://www.navitron.org.uk/search.php?search_in=all&search_text=frotter&x=17&y=11

You need to search the forum, not the shop!

Sadly, it apparently doesn't provide a full URL for the search:
http://www.navitron.org.uk/forum/index.php?action=search2 then type in frotter

Many thanks for everyone's replies.

To pick up a few points.

Yes we have batteries in the system as well. We have been through 3 stages:
1. Generator only, running for 18 hours per day, no electricity at night, very expensive to run
2. Generator + batteries, generator now only needs to run about 5 hours per day, so huge running cost saving
3. Generator + batteries + PV, in the summer we hardly need the generator at all but it will definitely be needed in the winter.

I found frotter in the Navitron forums. BTW if you search frotter on Urban Dictionary you go way off topic. Don't go there if easily shocked.

Exhaust heat recovery (which I hadn't thought about before) sounds a lot easier than trying to plumb into the generator's water cooling system. As someone pointed out, the water cooling system is pressurized (try taking the lid off the radiator when the engine has been running). Also it runs on 50% antifreeze mixture - might need quite a large volume of this stuff if it is going to go all the way into my thermal store. Would still need radiator / fan for times when the thermal store cannot accept more heat. So perhaps looking at changing the fan-belt driven fan for an electric fan linked to a thermostat. All do-able no doubt but beginning to sound like a major project to someone who doesn't have working on engines as a hobby.

Exhaust heat recovery sounds a lot less invasive, and I found this:
http://www.ejbowman.co.uk/products/ExhaustHeatExchangers.htm
which looks quite simple to add to the exhaust system.

No-one has commented on the length of pipe (generator is at least 70 metres away from the thermal store). Does anyone have any experience of piping heat over this kind of distance?

Yes 70m seems along way and expensive to insulate if you want any heat at the end.

For getting heat out of the engine cooling system - perhaps it might be worth looking at marine engines. these have a heat exchanger to dump heat to sea water. Unfortunately as with all things marine they tend to be expensive because they have to cope with salt water, but kits are available for many engines. This could give you the separation between engine cooling and heat transfer with enough design capacity and and thermostat control to cope.
Its a good idea but I have a feeling that the distance will sink it on economic grounds and moving the engine close enough may have noise implications??
Peter

Posted By: Joiner"No-one has commented on the length of pipe (generator is at least 70 metres away from the thermal store)"

Eeeek?http://1.2.3.12/bmi/www.greenbuildingforum.co.uk/forum114/extensions/Vanillacons/smilies/standard/shocked.gif" alt="" title="" >

Ecoflex by Durotan do standard CHP insulated pipe but its not cheap from memory approx £30 per mtr but it does work.

Joiner

Ok I realise Ecoflex is at the bottom of the market using plastic pipe but I dont want to give Andrew a heart attack when he finds out the price of the insulated steel in steel which I use at 230C.

Many thanks again for all comments.

It sounds like exhaust heat recovery plus insulated pipe (underground) is the way to go but I think I will need to do some financial calculations to work out whether this is a cost-effective thing to do. Moving the generator closer to the house will cost a certain amount of money but maybe less than the cost of insulated pipe.

We have only had the photovoltaics since May 2011 so I don't really know how how much generator use we will have over the winter. I already log generator-hours on a weekly basis, and I'll continue to do this. Then in spring 2012 I'll have some good data from which I can attempt to make some calculations as to whether heat recovery would make a worthwhile contribution.

Very useful forum. Thanks again for all the help.

Two analogies with small marine diesels spring to mind;

1) Calorifiers are standard practice for extracting heat.
2) Water-cooled exhausts are common both for cooling and silencing.
In a boat the exhaust water is wasted, but it could presumably be collected in a suitable tank and re-used. It would be pretty corrosive, so would need careful choice of materials.

The Italians seem to have done a lot of work in this area (microgeneration CHP) and I found a lovely little paper that ought to serve as a model on how to present report findings. Was tempted to link to it on here, except that it was pdf and anyway most people don't seem to have the time to actually read linked documents, more's the pity.

I can certainly see the advantages of microgeneration because it's an on-demand system that could take a building (and in theory a community) completely off-grid, with all the advantages of wind and pv (low or near-zero carbon depending on the biofuel used) without the inconsistency of wind or sun. Community-sized plants could easily supply all the heat and power for a number of buildings. In theory, you wouldn't need a power station of any description because demand would be wholly independent of any central supply or distribution network.

With a choice of whatever fuel type is most easily sourced locally.

Breakdowns could be covered by portable units, which could also provide temporary power wherever and whenever it's needed.

Anyone care to poke a few holes in that?