h a l f b a k e r yA riddle wrapped in a mystery inside a rich, flaky crust
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Typical home heating systems (boilers and hot air systems) are between 70 and 95 percent efficient. That is, they convert between 70 and 95 percent of fuel energy into usable thermal energy for home heating.
Why not use some form of heat engine (Turbine, Reciprocating, Stirling or even Fuel Cell)
to provide either mechanical or electrical power to run a heat pump (either air or ground sourced)?
One would use the waste energy from the heat engine in addition to the heat pump output to provide the thermal energy required to heat the home/business.
In theory, and I do mean _in theory_ we could increase the fuel efficiency from a maximum of 95 percent to maybe 120 percent or higher. Please don't get angry at the use of a >100 percent efficiency number, this is indeed possible in that the excess comes from low-level heat in the immediate environment.
A back of envelope calculation:
Total Efficiency = p2.(1-p) + p3.p.C
where:
p = efficiency of heat engine
C = coefficient of performance of heat pump
p2 = proportion of waste heat usable
p3 = coupling efficiency between engine and heat pump
The Total Efficiency is the number of units of heat energy from each unit of fuel energy.
One would hope that both p2 and p3 would be close to 1.
Some illustrative numbers.
p = 30%
p2 = 90%
p3 = 90%
C = 4
> E = (1-0.30)(0.9) + (0.9)(0.30)(4)
> E = 0.70 + 1.08
> E = 1.78 / 178%
One would probably need to use under-floor heating or possibly hot-air heating, since the heat output would be low level, maybe 30-50 °C / 86-122 °F.
Heat Pump Efficiency
http://en.wikipedia...eat_pump#Efficiency
Can be more than 100% [TheLightsAreOnBut, May 30 2007]
Whisper Tech heat and power generator
http://www.whisperg.../main/dcwhispergen/
Advanced and quiet heat and power generator [Pellepeloton, Jun 01 2007]
[link]
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This is an extension of the CHP (Combined Heat and Power) idea. CHP can be considered as an electricity generator with by-product heat used for domestic heating - but equally you can consider it as a boiler that produces electricity as a by-product. When the by-product electricity exceeds demand for electricity, you could use it to pump heat and somewhat reduce the fuel burnt in the boiler. |
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Whether the extra hardware required would pay for itself (in energy or cash terms) I'm not sure. It might if you wanted the heat pump anyway, for example for summer cooling. |
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So now we can use the electricity when that's in surplus - but I'm rather at a loss about what to do with excess heat when electricity demand is high, but heat demand is low. In places where this is a small proportion of the time, you might dump it wherever you're going to source the heat you're pumping at other times. |
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Well one thing that some low energy usage houses are starting to use is a system whereby a heat pump is used to suimultaneously heat water and cool a fridge (hot and cold sides). You can get many times the input wattage in effective heating and cooling power. |
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Add this to your idea and bingo.. |
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The "lost" heat in a home furnace, especially an ultra high efficiency unit is the heat needed to exhaust combustion gases from from the home. Be sure to include a method to avoid asphyxiation in your system. |
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[Galbinus_Caeli] ??? I've not seen a boiler design in years that doesn't have a balanced flue - the air used in combustion is drawn in from outside, and there's no connection with the indoor air at all. You still need some method of ensuring that the exhaust gases escape, or the fire will go out; but they can't end up inside the house. |
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You need very little power for a fan on the incoming (cold) air, which is the usual technique (possibly universal, I don't know), and the exhaust gases can be cooled as much as you like - although there comes a point where the cost of extracting heat from them is more than the heat is worth. |
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My less than a year old system draws from inside air and exhausts convectivly. It is an 85% efficient unit. (I put more money into the air conditioner, it gets more heavily used.) |
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And one of my furnaces is below the house so it has to exhaust air up two stories and out the roof, There are also feed branches into the chimney from the second floor (water heater) and attic (second furnace). Without enough flow, I could easily get backdraft. |
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Where on Earth do you live? I suppose the air conditioner comment should give me a clue :) |
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I live in Atlanta, Georgia, USA. |
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I should have mentioned that they could not even have installed a 95% efficiency unit under the house because of the length of the chimney. |
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And don't think this is because I have cheap units. I learned better, these are high end residential. ($500 more up front, $5000 less in lifetime repair costs.) |
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[Galbinus_Caeli] No, I understand. The economics of heating are quite different where the climate is such that you spend more on your air conditioning! |
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Our domestic air conditioning consists of windows we can open, and a few fans that get used a few times a year. |
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A heat pump only works when you have heat to pump. How are you going to pump heat out of -30°F air? I would think the energy loss from extracting heat from sub zero air would be far greater than a 95% efficient natural gas furnace. |
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lets say you use a Stirling engine to run a heat pump which moves the heat from the cold side of the engine to the warm side, the engine its self is only 85% efficient so it would loose 15% of the heat energy unless the heat pump extracts at least 15% more heat from the air in which case it would be running on whatever the heat from the air came from. |
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[Livingfishguy]: -30 might feel like it has no heat in it to us meatbags (thanks [pertinax]), but it is actually fairly balmy when compared to absolute zero. |
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heat pumps are pretty nifty devices in that they can move a significantly larger amount of thermal energy using x amount of electricity than could be created by using x in a heater directly. |
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for example, it is common to see heat pumps with 300% efficiency. you may, out of interest, want to look up COP and carnot. |
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[TIB]: Yeah, absolute zero would be pretty dang cold but the first point i was trying to make was that in climates like mine were in the winter it gets as cold as -30 on a regular basis, often getting even colder, a normal furnace would be more efficient than a heat pump because heat pump performance drops as the temperature difference increases, droping to 1.0 at about 0°F |
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My second point was that the extra heat comes from the environment so you would actually just be heating your house with solar or geothermal power. |
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<pedant>*us* meatbags (indirect object)</pedant> No offence intended; it's just my knee-jerk reaction. |
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[Livingfishguy]: interesting, and thanks for the clarification. |
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it turns out that a stirling heat pump would be quite a bit more efficient than the average vapour-compression-cycle version (reverse-rankine), which really makes me wonder why this technology isn't widespread. in fact, it is rather insensitive to outdoor temperatures and can maintain a COP of at least 2 at 0 fahrenheit. the other solution for really cold areas (like up here in canada) is to use the earth as a heat source/dump. |
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a friend of mine has a unit that does what you describe - it works as a heat pump until the temperature differential is no longer efficient (with primitive reverse-rankine) then switches to natural gas furnace mode. pretty good solution i guess. |
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Well now that i think about it, if you were to design some sort of highly efficent heat pump + heat engine device it would be theoretically possible to extract mechanical energy from anything warmer than absolute zero. |
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This is already performed quite will on larger scales, with coal, gas or garbage fired power plants dumping their waste heat into the surrounding neighborhoods through a water distribution network, and those living farther from the power plant heating with electric driven heat pumps. This works especially well in cold climates, like the Nordic countries, where peak electricity demand coincides with peak heating demand. |
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For areas with temps of -30, I would suspect that a ground source heatpump would be preferable to an air source. |
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