I don’t like heat pumps. Wait, let me rephrase that: I don’t like grid tied heat pumps. It’s the electricity! Heat pumps are an amazing technology which I am currently obsessed with, and I am in fact in the process of building one to heat my motorhome this winter. It will be grid tied. I am not doing this to save money, for the propane I heat with now is much easier, and cheaper. The heat pump won’t pay for itself in dollars and cents, but it will teach me a great deal about the thermodynamics and construction methods, and that – is priceless.
Electricity is an amazing medium to convey energy, but the way we utilize it now, that is all it is: a medium. Electricity is not a source of power, it is a means to convey power from coal fired steam plants, hydroelectric dams, wind turbines, nuclear fission, and a few other sources. Due to the economy of scale, these sources are able to operate at reasonably high levels of efficiency, and convert potentials (chemical energy, water pressure, air pressure, etc.) into relatively high voltage potentials, which are transmitted across vast distances to power our homes.
Electricity is an amazing medium to power lighting, electronics, charge batteries, and intermittent home appliances. When it comes to heating a home, I’m not so sure that grid electricity is the way to go. Resistive heating elements are an atrocious destruction of exergy, and will not be discussed further. Electrically operated fans and pumps for a biomass or fossil fuel system are reasonable, and may be low enough in wattage to be powered by off grid, home power generation systems. A heat pump, on the other hand, is not likely to be powered by an off grid system. Their consumption is just far too large, and although a fraction of a resistive system (I said it again), not small enough to be powered by PV or wind.
But wait! This doesn’t mean that heat pumps don’t have a role in my household, it just means I’m exploring unconventional ways in which to utilize them. For many years, I’ve wanted to build a home power system to produce electricity, heating, cooling, refrigeration, and possibly water purification. Wind is unreliable, PV panels are too costly to manufacture (in terms of $ and to the biosphere), solar thermal could be made to work, but produces poorly when needed the most, and hydroelectric is rare to possess, and is also fraught with environmental concerns. Fossil fuels are insane, and polluting. I used to both mine coal for a living, and burn coal for my home. It is dirty, stinky, and finite.
What does that leave? Biomass. If I could grow some sort of biomass and harvest it in a “sustainable way”, I may find a way to convert that stored sunlight energy into the power needs for my home. This project will take many years, and much hard work, but I feel it is the culmination of my interest in thermal energy systems, for it can include heating, cooling, refrigeration, dehumidification, distillation, electricity production, and of course heat engines. The source of biomass might be coppiced trees, fallen timbers, scrap wood, agricultural waste, fast growing grasses, or any combination of these or others. High efficiency combustion would be acceptable, but I lean towards a partial combustion bio-char method which traps some of the carbon into a porous substance that can be returned to the Earth as a soil enhancer.
Although some may view biomass energy as a low tech step backwards, I think it could very well be a step forward, when done responsibly. Operated outside the profit model, it might even help tie our culture to the health of the biosphere. Then again, our food system should do that, and people are growing further and further disconnected from the Earth’s natural systems.
The conversion of biomass to useful power might be accomplished in many ways. Thermoelectric is too expensive. A Stirling cycle would require large, cumbersome machinery, but it would be neat! No, there are only two methods I see as viable right now: a steam engine or gasification to drive an internal combustion engine.
A biomass fired boiler could drive a modified internal combustion engine in order to provide rotatary power. This seems like the simplest option. I lack any real machining skills to build my own small steam engine, and I’ve seen some neat engines modified for steam. I don’t know how efficient they are, but it might be worth looking into. I’ve also seen some small turbine systems (including Tesla turbines), but the machining required and the high speeds make my shy away from this option.
Gasification of biomass (or coal) is nothing new, and has been used to drive internal combustion engines since at least as far back as WWII. Because internal combustion engines are common, and it wouldn’t include the danger of a pressurized steam boiler, I would consider this as a viable option as well. Typically, these gasification systems are not set up to produce biochar, but I believe I have seen some experimenters messing about with such an idea. Two drawbacks, are the complexity of a gasification system, and dangerous carbon monoxide.
As I intend to power multiple systems with this machine, I am inclined to use the heat engine to drive a fairly large flywheel. The flywheel would act as a kinetic store of energy, and mediate changing loads imposed upon it. The drive system would power, at the very least, a 12 volt DC generator, and a refrigeration compressor.
Because this system will not be run constantly, there will be a need to maintain a battery bank, similar to any typical off-grid electrical system. 12 VDC is a popular choice, as there are many lighting and appliance choices built for such a circuit. The battery bank would need to have a capacity sufficiently large, so as to provide for the electrical needs between charging runs.
Some 120 VAC devices could be run through an inverter on the battery bank, but I may also find an advantage to adding an alternating current generator to the heat engine transmission, to be used periodically, such as when operating many shop tools. A gasoline or propane powered generator is a simple option as well.
Since I’m dealing with a heat engine, heating a home doesn’t take too much imagination. It would be preferable to keep the furnace in an outdoor shed or something, in order to keep the mess and the danger out of the home. A water circulating loop is most likely, with either a heated floor or a radiant floor system. I would like a large thermal storage tank that would act as a thermal battery to be charged periodically, more so in the coldest winter months. Domestic hot water is a simple byproduct.
On a more complex scale, a thermal generator driven ejector system could be built which would utilize some of the potential difference between the boiler temperature and ambient, to pump some heat from the outdoor environment with a refrigerant lowered in pressure by the ejector. I won’t go into it much more than that. These are not simple heat pumps, and generally provide low COPs. They could however lower biomass fuel consumption nonetheless.
Although I view air conditioning as an unnecessary luxury, in a poorly built or located home, it can be really nice. Because the home I intend to build will be small and designed to need little cooling, there may still be a need for it, as the weather is unpredictable if nothing else. As there are other functions in my home energy machine that will certainly need cooling, I intend to build a cold battery to go along with the hot one. I have long sought a system with a large ice storage means where systems that require cooling can dump their heat. A vapor compression pump could be coupled to the flywheel of the main drive, and be made to pump heat from a large insulated vessel of water, making ice. There are several methods the evaporator could be designed, but instead of a big ice block, I favor a batch ice system which makes flake ice or ice sheets. A batch system keeps the thickness of the ice minimized, and thus the evaporator pressure need not be excessively low.
In the warmer months, the rejected heat may be utilized in domestic water heating, simply rejected to the ambient air, or rejected to as radiation to the upper atmosphere.
In the coldest months, when the ambient air is below freezing, a passive heat pipe system could be made to keep the block frozen.
Ah refrigeration – my favorite subject. In my opinion, no home would be complete without a refrigerator and freezer. Although a compressor could be run directly from the flywheel main drive, the constant need for refrigeration, and the intermittent nature of the heat engine, makes this arrangement less than ideal. A low temperature phase change material could be found which melts near zero degrees F, and allows the freezer to stay cold for a lengthy period; I know of no such substance currently. Instead, I thought a well designed, well insulated refrigerator / freezer could pump heat with a small DC compressor, powered by the battery bank, and use the ice – cold sink as a condenser, maintained by the main drive compressor (a cascading system). This arrangement would allow the use of a very small DC compressor, since the condenser pressures would be so low, and the potential COP so high.
Dehumidification may be done in a similar fashion, but a small circulating brine loop and fan, could be a better option.
Frankly, to have to fire up a system like this on a daily basis, might be a real pain in the ass. I’ve heated with coal and wood in the past, and it wasn’t so bad, but the alternatives were very expensive, leaving me with few options. Because it is an ongoing research project, I may find it to be quite fun to play with my steam engine power plant everyday. Without a doubt, the system would be constantly improving, with better controls, and longer periods between firing. Winter heating might require a near constant fire, but little need to run the heat engine constantly.
Biochar production tends to be a batch process, so that in itself might be a drawback. A steam engine will likely take a while to build up pressure, which is a drawback there too. This is all very preliminary, and I’m just toying around with ideas at this point.
I should have put this section first, for it is the most important. The best way to solve our energy needs, it to use less. Use less water. Build smaller houses. Position our houses for the geography and the climate. Insulate and ventilate well. Build more efficient machines that are designed to just barely meet our needs, so that we adapt our lifestyles to work within their limitations. I can’t stress the last point enough.
Oh yeah, and retire this dysfunctional culture, putting an end to infinite growth economics.
Pay attention to your impact on the world; it’s more than you think. I don’t wish to lessen my impact, I want to increase it – in the positive direction.