The units can run at something like 80-90% efficiency (in terms of power out to potential energy of fuel in), although that needs detailing. Will try and find that little report I mentioned earlier because if not actually food for thought, it was a very appealing looking appetiser!

It just all seemed to make so much sense. Micro-cogenerating plant somewhere in Italy is still running 30 years on.

If distribution losses are around 9% and overall power generation efficiency is at the peak that a gas turbine/alternator combination can achieve (around 60%) then it looks as if centralised power generation and distribution converts about 55% of the potential energy in fuel to useful power at the point of use. By any rational standard this is pretty poor.

If we all switched to local (either neighbourhood or household) combined heat and power systems, with the excess generation heat in summer being used for absorption refrigeration/air conditioning, then it seems likely that overall fuel-to-usable-power efficiency could be around 80% or better.

The next question is whether this approach is cost effective. Certainly there would be a significant saving in centralised power generation cost, plus a massive saving from gradually removing the power distribution network. There would also be a significant impact on the visual environment by the removal of the electricity distribution infrastructure and reduced impact of power failure (because it would be very localised). Would the increased cost of distributing fossil fuels, or renewables like wood pellets, waste etc, be more than or less than the saving though?

My guess is that we probably need a combination of local combined heat and power generation, with a reduced capability national grid, plus local PV, wind (where appropriate) and direct solar for water heating. Combined with reduced energy housing we could probably halve our domestic energy usage (in terms of fuel consumed to produce it) within ten to fifteen years. The challenge would be enabling it to happen. People are not going to want to replace gas and oil boilers with CHP units unless there is a good incentive (a bit like FITs). Whether the country can afford to stump up the capital needed up front, in order to see the benefits in future years is debatable.

<blockquote><cite>Posted By: DamonHD</cite>No, 55% overall is pretty good given the Carnot limits.

Don't confuse heat with electricity: 1kWh of electricity can do far more work than 1kWh of heat under normal circumstances. (The fact that we waste it with widespread resistance heating and incandescent lighting is practically a sin...)

If centralised generation was that bad we simply wouldn't be doing it, and we'd back to the days of Tesla/Edison in NYC with generators on almost every block. Those greedy bean counters / bankers / captains of industry can see which delivers the most bang for their/your buck.

Rgds

Damon</blockquote>

As a retired scientist I can assure you that I don't get so readily confused! As a developer of high efficiency electric vehicles I am reasonably familiar with the subject of electrical power systems and power conversion. What I can tell you, with absolute certainty, is that 1kW of electrical power delivered to a resistive load will deliver 1kW of heat energy, so is 100% efficient at the point of use. It may be that we cannot usefully use all of that delivered heat energy, but that is a different matter.

Also, the demonstrated total efficiency of CHP units is around 80% or more, not 55% (using the potential energy in the fuel to the useful heat and electrical energy delivered to the end user).

<blockquote><cite>Posted By: DamonHD</cite>Nope, 1kWh of electricity delivered to a heat pump could in turn deliver you several kWh of heat (I'm talking house temps here, not furnace), so though the resistive heating is nominally 100% efficient it is simply a waste of most of the available energy.

Rgds

Damon</blockquote>

Did you read my post? It appears not..........

I specifically mentioned a resistive load, not a heat pump. Carnot's Theorem refers specifically to heat engines doing work, BTW, not the total energy efficiency of a system that uses energy recovery from the wasted heat for a purpose other than doing mechanical work, so it is incorrect to quote this as an absolute efficiency limit.

A CHP plant significantly exceeds the efficiency (in terms of usable energy from the potential energy in the fuel) than that of a simple heat engine delivering mechanical power to a generator, because although the heat engine efficiency is still only around 40 to 45%%, waste heat recovery for direct/water heating more than doubles this in terms of overall efficiency. This is practical for a small to medium scale CHP plant, but less feasible for a large central generation plant where utilising the volume of waste heat is much harder, due to distribution costs and losses. Drive past any power station and you can see this massive waste heat, in the form of vapour rising from large cooling towers.

FWIW, many big power stations today use a form of exhaust heat recovery to gain efficiencies of around 60% or more and exceed the normal Carnot limit for a simple heat engine. I did mention this in the previous post, but perhaps you missed the CC reference. CC stands for Combined Cycle, and in essence is the technique of using gas turbine waste heat to generate steam that then runs a steam turbine, so increasing overall fuel efficiency.

BTW, if you take the time to study the physics of such systems you'll find that they potentially have the capability to run at around 95% efficiency, so it is quite reasonable to assume 80% efficiency as a starting point.

Efficiency is such a difficult term, what is really being discussed here I think is Exergy, or usable work from an energy input.
I could easily make a CHP system that captures nearly all the heat the motor generates, add in some batteries, capture the heat from them as well, but then what would I do with it all, especially if my modal temperature was lower than what I wanted (why you can never apply means to an individual case).
I am interested in high efficiency electric vehicles though, one way to make them efficient is to reduce the mass, they would not go very far though, but if you measure 'efficiency' by mass times velocity carried per unit energy input it would look pretty good, but if you did the same thing with half mass times velocity squared it would not look so good (one is momentum and the other is energy), both are valid though.
Covariance also comes into it, the further you are from the energy source, the less affect it will have on you, so it is also possible to have a small local supply have a larger affect than a large distant one, or the other way around it just depends on the sizes and distances.
If people are seriously considering CHP then they have to take both electrical and heat storage even more seriously, and to my mind this is the stumbling block. Dead easy and cheap to make a small 5kW(e) and 8kW(h) CHP unit that would service most domestic dwellings, probably do it for about 500 quid installed (can get a 7.5kW(e) for under 300 retail), but then if you said you need a thermal store at a grand before fitting, a suitable battery bank and inverter for about the same, then you can only run it on an 'approved' fuel, like pump diesel or natural gas, I think the whole concept, with today's technology and infrastructure is a non starter.
But I really think this a subject for a new thread, send me the link as I am tired at the moment.

Spot on, Steamy Tea, and exactly the way I've been looking at CHP for some time, except I think it's possible to do now, with existing technology.

The main problem with centralised electrical power generation and nationwide distribution is that getting on for half of the potential energy in the fuel is wasted, either as heat directly emitted from the power stations or as heat from the distribution network. Given that a significant amount of the delivered electrical energy is turned into heat at the point of use it seems crazy to have such a wasteful system.

Imagine this:

- New houses have a low power CHP system, combined with a few kWp of PV (with energy storage), some solar DHW and a low power grid connection.
- They have large daily thermal store for hot water and are built on a massive seasonal thermal store that is heated by the CHP heat output.
- They have supplementary heating that is provided by a heat pump, drawing heat from the seasonal thermal store.
- They are super-insulated and operate as passive houses, only needing modest supplementary space heating.
- All lighting and appliances are optimised for maximum efficiency and minimum waste heat generation.
- Refrigerators and freezers run on waste CHP heat or solar heat, primarily using absorption cooling, with a minimal use of conventional compressor/evaporator systems.

The result would be a significant reduction both in fuel consumed and CO2 produced, for a given standard of living. With energy costs dominating house running costs for many, the saving through life would be substantial, particularly if the cost of systems like CHP and PV fall with increased production. As CHP can, at least in principle, run on pretty much any fuel there may even be the possibility of getting consumption low enough to make renewables an option for that, too. At the moment, with our high energy usage per household, burning wood is barking mad, as it simply isn't sustainable as there isn't enough land available (how many acres of woodland per household would be required if we all used wood as fuel?). Cutting household energy back by a massive amount may may this equation look a little better.

I'm getting lost. Who's agreeing with who now?

Example of how easy it can be....

http://www.cogenmicro.com/index.php?select=160

And for an analysis of the current situation and prognosis (dire, given the lack of political will, hardly surprising given the sheer volume of lobbying by those wanting their share of the heavily-subsidised wind-rush)...

(A sizeable chunk of download, even if free to download, but there are summaries available at certain points)
http://www.springerlink.com/content/978-3-540-25582-6/#section=412286&page=2&locus=16

And this is going to either please or upset a few on here, although if you take a more neutral stance it gives a clue as to what's informing current thinking on FITs - allegedly...

http://www.cospp.com/articles/print/volume-9/issue-5/features/feed-in-tariffs-for-micro-cogeneration-how-to-maximize-carbon-benefits.html

There is so much out there, I cannot see why there is so much scepticism about a wider application of the technology. What is everyone so afraid of? The technology is there - proven - and the sums have been done and found to add up. Wherefore?