It’s a marriage made in heaven: Solar PV and Geothermal Heat Pumps
Part 3 of a 6-Part Series
Geothermal Heat Pumps, the Harry Potter Under the Stairs
Is there a
- renewable technology that
- uses electricity
- efficiently to heating indoor spaces?
Yes, the trifecta answer is the geothermal heat pump (GHP). It runs on electricity, it’s the most efficient heating system available and it’s renewable.
The major difference between an air source heat pump (ASHP) and a GHP is the medium of exchange. Instead of transferring heat via the air, the GHP moves heat to and from the ground. This heat comes from the sun, for the ground absorbs almost half of the sunlight and heat that falls on it. This is why the term “geothermal” (Greek for “Earth heat”) is used to describe it. Another name is ground source heat pump (GSHP).
GHP technology is underground metaphorically and in reality. Few people are aware of it since it lacks the visibility of solar panels. The GHP uses an underground heat exchanger to store or withdraw warmth. It can exchange heat via an open loop (water from wells) or a closed loop (fluid circulated in buried loops of pipe) system.
While the air temperature in the Northeast can vary from well below freezing to the upper 90’s, the temperature in the ground below the frost line remains a stable 40-60° Fahrenheit year round.
In summer, ASHPs are less efficient in exhausting heat into the hot, humid outdoor air than GHPs are in depositing it in the cool ground. This heat is stored underground, with relatively little loss, until it is withdrawn. GHPs consume about 30-40% less electricity in the summer while providing a greater level of comfort.
In winter, ASHPs are less efficient trying to extract heat from the frigid air than GHPs in withdrawing what is stored in the ground. GHPs move three to five times more energy than they consume.
If this sounds a bit like a perpetual motion machine, then here is a review of the laws of thermodynamics. The first one states energy cannot be created, but simply converted from one form to another. It can change form when some type of work is done.
When a refrigerant gas is compressed by a heat pump (work) its temperature increases, releasing heat. For every watt of electricity used to run the heat pump, four to six watts are converted into heat, so the total thermal energy output is much greater than the electrical input.
The second law states heat naturally moves from a hotter body to a colder body. It takes work to go in the opposite direction. So a geothermal heat pump works with the natural dynamic of heat where an ASHP works against it.
As the seasons circle through the year, GHPs recycle heat, withdrawing it in the winter and depositing in the summer, with the ground acting as a suitable storage medium or “heat bank.” Wrapped up in one unit you have renewable thermal energy, energy efficiency and heat recycling.
Although the electric usage goes up in winter, there is NO fossil fuel bill for heating. The overall utility costs go down. In addition, GHPs can also provide hot water.
Homeowners love the comfort and low operating costs that come with GHPs.
The downside? There is an upfront cost to put in the heat exchanger. Although this initial installation expense raises the cost of the system, it is an excellent investment since the heat exchanger has a very long lifespan, estimated to be more than 60 years. The Commonwealth Building in Portland, Oregon, has one that has been operating since 1948.
Since most heating equipment installed today has an expected useful life of between 20 and 25 years, between 4-5% of existing systems must be either replaced or significantly overhauled every year. Why install a new heating system with an old technology that will be superseded before its useful life is over?
For such long-term investments, it makes sense to switch to a renewable system before an aging one fails. The smartest financial advisors should caution homeowners to abandon fossil-fuel systems and invest in a technology that will still be around in 2050.
Federal income tax credits to offset the upfront costs for GHPs are similar to those for PV systems. Although due to expire in 2016, there is a bill in Congress to extend them to 2021. New York State is poised to adopt GHP tax credits and sales tax exemptions similar to those promoting PV use, once Governor Cuomo signs the bills. The NYS incentives are in addition to solar PV, so there is no competition between the two technologies for them.
So far, only two electric utilities in New York State have opted to provide rebates for GHPs because they reduce peak demand. Support is growing for more.
Is California next?
Next Blog: Oversaturation of the PV Market