Heat Pumps + MicroCHP = DIY Home Generators?
When two complimentary technologies collide for the first time, sparks fly. The excitement comes at the prospect of generating heat and electricity by harvesting the heat out of the ground below your house.
For a number of years now we’ve known about heat pumps. They’re basically a refrigerator that moves heat from one place to another. Typically, heat pumps have a coefficient of performance (C.O.P.) of between 3 and 4 and can be as high as 5 if they’re geothermal. This means that with an 100% efficient electric heater takes 1kW one watt of electricity to create 3.4 BTUs of heat. However, with a C.O.P. of 5 a geothermal heat pump can harvest 5 times the amount (17 BTUs of heat for every watt of electricity).
That’s fun stuff, but now there’s a new technology gaining momentum in the market called micro combined heat and power (a.k.a. micro cogeneration or microCHP). These units burn natural gas to heat water to then power a generator that creates electricity. They typically convert 10 – 20% of the heat into electricity and the waste heat is used as hot water for your house. A side benefit is that the electricity doesn’t have to travel over transmission lines (no energy loss).
Hooking up a gas powered micro co-generation unit to a heat pump will without doubt get you more bang for the buck. You’d be able to use the electricity from the unit to power the heat pump and your overall cost would be significantly reduced.
What is still to be seen is whether the gas powered micro CHP could be replaced by a Stirling engine that runs on hot water. The idea being to have the geothermal heat pump generate the hot water to run the Stirling engine which then in turn generates enough spare electricity to run everything and charge a battery.
Updates > Earth Energy:
Co-generation (+heat pump) experiment using natural gas:
http://www.ccht-cctr.gc.ca/projects/earthenergy_e.html
Via Dave: How to generate electricity using thermal differences in the ocean:
http://hawaii.gov/dbedt/info/energy/renewable/otec
A great talk about Stirling engines used with solar collectors here:
http://www.ted.com/index.php/talks/bill_gross_on_new_energy.html
If you’d like to do this on a larger scale check out Solar Heat Pump Electrical Generation System. Unfortunately, this system does not use the waste heat to heat your house.
I’d like to know what the basis of your $30k estimate is for this equipment. My understanding is that many GSHPs cost upwards of $30k by themselves to install, and the only US based Micro-CHP system of which I am aware is close to $20k for installation.
When I installed a geothermal water to air unit in my house a few years ago it cost less than $15K. But you’re right, the costs depend on the size of the unit you’re buying and whether or not you have to drill through rock for a closed loop system. If you’re lucky, an open loop system or a nearby body of water such as a pond or lake will keep the costs down.
As I understand, the microCHP units range from $13K to $20K installed.
Of course, you could spend a lot more than $30K for both by the time you pay for installation, but I don’t think my estimate for the equipment is that far off. Plus, who knows if it would work? I’m just suggesting the possibility that it might.
"The new Honda MCHP Deluxe system features advanced energy management technologies for home heating and power combined with a new automatic back-up power feature (a residential stand-by generator) that can provide back-up electricity in the event of a power failure." Read more..
http://www.greencarcongress.com/2008/11/american-...
Hvis dette var muligt, ville evighedsmaskinen være opfundet. Der er koldt i jorden og varmt i huset. Varmepumpen øger denne forskel yderligere og det koster energi ligemeget hvordan du vender og drejer den. Producenterne pynter på tallene. Thermodymikkens love tillader det ikke. Beklager
If this was possible, then the eternity machine would have been invented. It is cold in the ground and warm in the house. The heat pumpe increases this difference and this costs energy. You can recover energy by reducing the difference again with, for example a sterling engine, but it can never be enough to drive the heat pump. This circle can never work.The manufacturers are beautifying their specifications. The laws of thermodynamics will not allow it. Sorry!
The ground might seem cold, but it’s relatively warm. The heat pump is simply transfering heat from one place to another. If there’s enough heat to heat a house, there’s potentially enough to heat a stirling engine also. This would not be breaking any laws of thermal dynamics. The heat that is in the ground is real and can be harvested. The question is whether or not you can break even or gain energy from the process and the answer is yes with the right system.
Jon, the specs for heat pumps and for micro-CHP are accurate and verifiable. I currently own an air-to-water heat pump to warm my pool. It works great and costs far less than heating the water using just electricity or gas.
I also owned a water-to-air heat pump and they really do harvest energy out of the ground or a body of water. They do use electricity to move the heat from the ground to your house both for the heat pump and for the circulation of the fluid in the pipes in the ground. The amount of heat that they generate is far greater than the amount of electricity that they need to run.
The micro-CHP is essentially working in reverse to produce electricity where 10 to 20% of the heat is converted to electricity. If you can use the waste heat produced to heat your house, the right system theorically could easily heat your house and produce electriciy. It’s a matter of efficiencies and it depends on how warm your source of heat is such as the ground or a body of water.
How can I explain this! hmm!
Off course I do not dispute that with a heat pump you can heat a lot more for the same electricity becaue you cool the ground and use that thermal energy to heat the house. The factor for this is the COP. If it is 3, you get 3 watts of heat for every watt of electricity you spend on the heat pump. The COP varies with the working conditions of the heat pump. The smaller the temperatur diference between the cold liquid from the ground and the hot liquid or air leaving the heat pump, the better COP you get. Fot the sterling engine it is the other way around. The bigger the temperature difference the better the efficiency, so what is good for the heatpump is bad for the sterling engine.
The manufacturers are giving specs for their equipment under reasonably favourable working conditions. You cannot take the specified COP of a heat pumpe and calculate it up against the specified efficiency of the sterling engine.
Thermal energy likes to move from warm to cold. The sterling engine takes advantage of this desire and turn some of this thermal energy into electrical energy and the whole mix gets colder than it would have been without the sterling engine because the sterling engine took som energy. The heat pump is doing the opposite. It goes against the desire to mix into one common temperature. Instead it increases the diference in temprature. This, in contrary to the other process, costs electrical energy to run the heat pump and the average temperature af the whole thing becomes higher, because the heat pump added some electrical energy.
The sterling engine is essentially reversing the process of the heat pump and it can never take more energy out of it, than what was put into it without cooling the house down and heating up the ground.
Laws of thermodynamics will not allow you to “harvest” heat from a colder place and put it into a warmer one with a net gain of energy. This would be necesarry if the sterling engine should deliver at plus in electrical energy by reversing this same process again.
I can never work. It is difficult for me to explain. I hope you will understand what I mean.
Heat pumps that use the ground as a source of heat (rather than the air) can produce a lot of real heat year round. Its source of heat was water in the ground. The larger the heat pump, the more heat produced.
MicroCHP power generators run by burning gas. This is a Stirling engine that converts heat into electricity. If we were to replace the gas heat source with hot water and if you attached a Stirling engine that could convert 20% or higher then you would be producing a surplus. There are Stirling engines that have efficiencies as high as 30%.
The important part to remember, is that the goal isn’t to generate lots of electricity, but to heat your house with the ‘waste’ heat that is left over. We’re not trying to get rid of that heat, we want to use it to heat a house.
You’re right that for Stirling engines, the high temperature differentials will increase their efficiency. The source of the heat going in isn’t cold air or cold water, it’s very hot water produced by the heat pump.
You have to remember that such a furnace will be running more often when it is colder outside. So you couldn’t produce energy year round unless you didn’t mind heating your house during summer (or say heating your pool).
I’m not making this stuff up, you can find these details out here:
http://en.wikipedia.org/wiki/Sterling_engine
http://en.wikipedia.org/wiki/Heat_pump
Patent found!
This idea was granted a patent to Baron Denis Philippe:
http://v3.espacenet.com/publicationDetails/biblio?CC=WO&NR=2007101919&KC=&FT=E
Yes! – You are making it up
If you want the heat pump to heat the house AND run a sterling engine you would need to add extra electrical energy to the heat pump. Even this extra electrical energy you would be unable to recover fully through the sterling engine.
Even if you got the sterling engine for free you would reduce the effectivness of the whole system by connecting it. A sterling engine has nothing sensible to do alone with a heat pump. It is only attempting to reverse, what the heat pump just did. You would save more on the elctricity bill by turning down the heat pump a bit and disconnect the sterling engine. You are attempting to invent the eternity machine. It is againt fundemantal laws of physics. It cannot ever be.
I cannot explain it. I have to realise that. I really tried – sorry!
Hi Jon, you’re quite right, as I understand, geothermal systems require much hotter water to create a surplus of electricity. Thanks for the comments.
@Jon Møller
You are right that with a single external heat bath (the ground water) you cannot do this – violates 2nd law thermo. However, if you have two external heat baths you can do it in principle.
for example, allow heat to flow from ground water to air. tap that flow to produce electricity. use that electricity to run heat pump from ground water to heat your house.
the problem with such a scheme is the delta T between ground water and outside air may not be very large, and so the elec generation is not very efficient.
Thanks Dave, that’s quite right.
There are a couple of things to consider for geothermal heat pumps, one is that the ground needs time to ‘re-heat’ so to speak if you’re using a closed loop system (filled with water or anti-freeze) The heat pump can harvest a lot of heat and ‘put’ a lot of cold back into the ground. However, if the system runs continuously the C.O.P. would drop (significantly) and the heat pump becomes less efficient at pulling heat out of the ground because the ground needs time to warm up around the pipes. As I understand, geothermal heat pumps are designed NOT to run continuously but only some of the time.
This is not the case for an open loop system. The temperature of the water going in is constant year round. Plus, the colder water that has run through the heat pump doesn’t go back into the same location. It might run down a hill for instance.
I’m pretty certain that if you match the right Stirling engine you can make the heat pump run from that electricity. You wouldn’t be generating an abundance of electricity, you’d probably barely break even. However, since the goal isn’t to make a power station, but rather to heat your house with all that ‘waste’ heat. If you happen to live somewhere with a flow of constant temperature water you should be able to harvest it no problem.
I also think it might be possible to set up a closed loop system to work continuously if you doubled the number of loops. There would have to be a timer that would switch on/off between each set of loops.
Of course, here I’ve been thinking about cold ground temperature up here in Canada which is what? 8°C or something close to that? Imagine if you did this down on a hot beach and ran the whole system in reverse? You actually could make a decent electric generator out that! I think I should write a new post on it.
@Jobe Roberts
i found this post hoping someone was thinking of a turnkey system that would work off ground (or well) water. not yet, is the answer. I was also worried about the “size” of the thermal reservoir and open loop vs closed loop. no idea what the answer is. as for the beach … here you go
http://hawaii.gov/dbedt/info/energy/renewable/otec
cheers