Heat Pump or Heat Engine?

I haven’t bent any copper for a few weeks now.  Between the heat, a vacation, and some bad habits, the shop has barely been opened.  Part of the reason is due to a recent fascination with power plants.  Previously, in The Power Plant -or- The Wood Fired Refrigerator, I discussed the possibility of abandoning vapor compression heat pumps for a biomass source.  My opinion hasn’t changed greatly in that time, but what is up in the air right now, is whether or not to abandon my current vapor compression work, and focus on heat engines instead.

As my work with ejectors stand, I feel that I’m making progress, and in time I’ll have a self regulating flooded evaporator.  The plan was to acquire a large outdoor heat pump coil and modify it for flooded operation.  A small hermetic compressor, supplemented by an ejector primary, would pick up heat from the outdoor environment this winter, and condense the propane refrigerant in a water holding tank.  This water would then be circulated via pump into a water to air heat exchanger in the motorhome.

As of this date, that basic design has not changed.  What has changed, is my opinion of domestic heating with grid powered vapor compression.  For the last day or two, I have been considering the abandonment of my heat pump, for the implementation of a small propane fired power plant.

The first technology considered, was a biomass gasification setup, which could provide a combustible gas to a small internal combustion engine – coupled to a generator.  The waste heat would then provide domestic heating, and possibly refrigeration.  For a variety of reasons, I find this technology to be flawed, and outside my level of interest.  The greatest hurdle, is the management of condensing tars which are produced from the pyrolysis of biomass; they tend to gum up IC engines.  Many methods exist to manage the problem, including high temperature cracking, but when I consider the limits on fuel size, shape, and moisture content, producer gas is out.

Another method of producing motive power is steam.  Biomass gasification furnaces can be built so as to provide a high thermal efficiency, and low emissions.  Instead of introducing the gas into an IC engine, it is burnt nearer the point of pyrolysis, where the heat is absorbed to warm, evaporate, and superheat water to steam.  The superheated steam (under pressure) is fed into a reciprocating engine, so as to provide the motive power to generate electricity, pump water, or power a fan.  Of course, the byproduct of this is useful hot water.

I doubt I will attempt this in the trailer park, but my research has me excited to the point where I am considering a propane fired model.  Since a propane burner would simplify construction (and alleviate the need to acquire a source of biomass), I figured I could generate electricity and heat water at the same time, utilizing the water to air exchanger from the heat pump plan.  Propane firing is not a long term model, but a research step towards eventual biomass gasification.

For power generation, the plant would almost have to be manned, so if it were only fired once a day, all of the heating needs of the motorhome would need to be stored for use until the next firing.  The other option being a lower temperature thermostat setting to maintain heating needs.

What’s clear, is building a power plant before the cold weather sets in is a very tall order.  Hell, the heat pump I had planned probably won’t be up and running by the end of the year either.  I need to be realistic and choose a path that makes more sense given my long term goals.  Under that construct, the power plant make more sense, with vapor compression used for refrigeration and air conditioning needs.  However, with the time and investment I have already put forward in my vapor compression research, it would do me well to continue with the heat pump project, for what I’ll learn the process is invaluable in itself.

As much as it pains me to set the heat engine project aside, I know much more about vapor compression, and I hope to attract some attention with that work, and hopefully a job opportunity associated with it.  Even though the heat pump is not likely to be an economical heat source, I am too heavily invested with that work to drop it right now.

Time to go bend some copper!

-M.C. Pletcher