Focus on Energy: QUESTIONS AND ANSWERS

Answers provided by Dick Hill, a retired emeritus professor of engineering at the University of Maine.

From George Shaler, Portland:

I have done some reading on geothermal heat pumps as a way to heat and cool one’s home. However, much of what I have read has come from other parts of the country. Can you tell me if you have come across any news on whether these units can be installed in colder climates such as ours?

Hill: In some places on the earth, like Finland, heat actually moves from the lower layers of the earth’s mantel toward the surface. Here one can place pipes in the ground and capture some of this energy for heating buildings and hot water. In Maine, the heat flux from the lower mantel toward the surface is something like one or two watts per square meter, not enough to do any good.

What we can do is use what is called a “ground assist” heat pump. The earth warms in the summer, and we can “pump” some of this heat into dwellings for useful purposes in the winter. But first a word on heat pumps

All refrigerators involve three simultaneous events: Something gets cold, something gets hot, and power comes in from the power company. (In a domestic refrigerator, the thing that gets hot is that coil on the back of the unit.)

A ground assist heat pump is a refrigerator. It cools the ground and puts the heat generated into the house. The important thing to know is the relationship between the heat we can get into the building compared to the power we need to buy from the power company. A poor system can obtain a two-to-one ratio between heat generated and power paid for. A good system could be as high as five-to-one.

Let’s try a ground-assist heat pump design. If we dig up the back yard and bury pipes in a section of the yard that is 5 feet deep, 20 feet wide and 40 feet long, we have 4,000 cubic feet of dirt as a “source” for our heat pump. Dirt is about 100 pounds per cubic foot and the energy we can get from dirt is about .5 Btu for each pound changed in temperature one degree. If we manage to cool all this dirt 30 degrees (say, from 65 degrees down to 35 degrees), the energy delivered to the house will be the equivalent of only 60 gallons of heating oil.

So that won’t work. We cannot heat our homes by refrigerating the dirt in the back yard. The university constructed a ground assist heat pump in Orono with 1,000 feet of 1.25-inch-diameter black plastic pipe buried in the ground and filled with antifreeze solution. At the end of the first week -this would be in September -the solution being circulated in the ground fell from 50 degrees to 25 degrees. For the entire rest of the winter, the temperature of the fluid circulated in the ground did not fall below 23 degrees. We were freezing a great cylinder of ice down in the ground. To cool a cubic foot of earth one degree yields 50 Btu; to freeze a cubic foot of water yields almost 9,000 Btu. The irony here is that during much of the winter the outdoor air temperature is above 23 degrees. Why go to all the trouble to bury pipes, when one can just cool off the outdoors?

The relationship between heat delivered to the dwelling and the electricity needed to deliver that heat depends on the temperature of the zone used as a source of heat. One could use outside air as a heat source down to 23 degrees, then shift over to the pipes in the ground when the outside temperature is below 23 degrees. But that gets complicated.

Heat pumps (solar assist, ground assist, deep well, or whatever) must become the mode of home heating in the future.

From Hugh S. Cowperthwaite, Portland:

I have a question and appreciate you writing about this topic and soliciting questions from the public. I have a natural gas furnace that is efficient, but expensive to operate. I am looking to install a second heating system as a backup and as a way to diversify my fuel dependence. What are the different choices, and is there a way to compare their overall costs? Does it make sense to diversify, or should I be 100 percent reliant on the one source of fuel to heat my home? If I have a natural gas furnace, should I install a kerosene monitor-type heater that is more efficient and would also give me a choice of what fuel I want to burn when it’s 40 below out?

Hill: Your central gas system has significant losses in duct work and fans. The monitor “puts the heat where you are” and will save much of the distribution energy loss of the central gas system. This is particularly true for the low-heat demand portion of the heating season. During the next month or two, when one wants to get the chill off for the second cup of coffee, the monitor is a better bet than cranking up all that duct work from the gas furnace. It will also supply some fuel diversity, which should help.

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