Hi,

     I have used propane to heat my 4500 sq ft greenhouse before the cost of propane went up. Costs were over 1000 dollars per mo. during winter months. Since propane costs have more than doubled since then, it eats up a lot of profit. Natural gas prices will soon be rising also.
    One idea that crossed my mind is to enter into an arrangnent with a tree trimming company to recieve their wood chips.  Auger them into a silo next to a coal style boiler. Auger the dried chips into the boiler after they have had time to dry.  The silo could be fitted with an air line to push drying air up through the silo. The older dryer chips would be on the bottom while the newer green chips would be on top.  Corn stalk chips are also a more costley  option.

     Any liquid fuel boiler would give you the option to use what ever liquid fuel is least expensive.     
       

    The boiler water should perhaps contain antifreze to lube the recirculation pump, and etc.
     Getting the combustion out of the greenhouse also eliminates the possibility of flue gasses leaking into the greenhouse.
               good luck to you
               Sincerely Ray Peterson

Does anyone have cost figures for heat pumps in greehnhouses?

One quick formula to figure yearly fuel consumption is based on the heating degree days of the location.  The formula is ( 24 x design heat loss x HDD ) / ((T inside - T outside) x btu content of fuel x efficiency of heater)
  

If using heat pumps the EER of the heat pump is the ratio of the btu/h output of the heatpump  to the electrical consumption of the heatpump.  So divide the above number of yearly heat consumption by the eer and that will give you the yearly electrical consumption.