Andrew Heggie wrote:

On Sun, 04 Apr 2004 08:08:48 +0100, The Natural Philosopher
wrote:



No. Turbines are very effecient beasties. More efficient than stirlings.


Yes but stirlings aren't at all efficient in converting high grade
heat to movement. Their potential lies in cleanliness and long working
life.

I think diesels still win hands down on anything except combined cycle
systems, and they can also be enhanced by this. High speed diesels
will exceed 40% conversion of heat to electricity, low speed ones
(burning even cheaper fuel) nearly reach 50%.



I thnk you are optimistic there frankly.


It's scalability that's difficult with CHP. None of the current
generating technologies scale down well or have particularly good
performance when turned down. Small alternators are inherently less
efficient than larger once because of engineering tolerances and
magnetic losses.



I beg to differ. Its not the alternators that are the problem. I can
show you a 2 oz generator that is at least 80% efficient. Its the
engines that drive them.



"Good" chp systems seem to feature loads in the MW levels and minimum
loads approaching 1/3 of peak loads. They feature multiple engines
running in their peak efficiency regions, as loads increase more
engines are brought online, the price is the higher O+M costs of
reciprocating engines. They also make use of both the coolant and
exhaust heat, for heating and cooling via adsorption coolers. As
someone else said more electricity is used worldwide for cooling than
heating, intuitively this is because most heating is by non electrical
means.


I actually doubt the above, on nearly every point.

The reason that generating sets are usually in the MW capacity is that teh

capital cost per megawatt is lower for larger sets. Not efficiency per se.


Building a big condensor takes about as many man hours as building a
little one. So costs do not scale lineraly with size.

When CHP in toto is looked at, if you can utilise the waste heat for
something that saves electricity, inefficiency in the thermo-electrical
conversion is not so serious. Water at 30C is almost useless for
extracting mechanical energy from, but makes fine underfloor or
undersoil heating for e.g. greenhouses.

I also challenge the 'more electricity is used for cooling than heating'
- this may well be so in e.g. the south west USA, but in my house the
electricity all goes into heat actually - lights, cookers, computers -
it all ends up heating the house. Ultimately all electricity ends up as
heat one way or another, or in the construction of e.g. some material
like Aluminium, which represents a potentially burnable material that
would release iheat if burnt.



At the MW(e) level you are buying your prime energy at industrial
rates, which will be half to a sixth what a small domestic user will
be charged.



I think not. I think the base cost of generating in the most efficient
sets is around 2p per Kw/h. Not far off domestic night rates.


AJH