Question

Are geothermal heating/cooling systems better in reducing energy costs than solar power?

Asked by Kimberly

Which would you recommend for areas subject to hail storms and high winds?

Answer

Mick Dalrymple

Answered by Mick Dalrymple

Scottsdale, AZ

Eco-Friendly Building Center

June 26, 2008

Great question that gets to one of the roots of my approaches to sustainability.

First, though, let’s talk about hail and wind. Solar panels must pass a 60-mph, 1-inch hail impact-resistance test to get their UL listing. So, if that doesn’t cover your situation, I can suggest a few better climates to you for possible relocation.

As far as wind, your solar photovoltaic (PV) or solar thermal contractor will need to design a mounting system that passes local code for your climate conditions. You can request that they go above and beyond code if you are not comfortable that code is sufficient. The panels, in themselves, do not have a problem with wind. The issue is the damage that they may cause to your roof, to another structure or to people if they take flight off of your roof. A geothermal system is not going to be impacted by hail or wind, as it is intalled in-ground and in the house.

Energy efficiency before renewable energy

Now to the bigger question: Sometimes PV colleagues get anxious when I say that, in my book, energy efficiency comes before renewable energy.

An analogy in healthcare is to treat the cause instead of focusing on the symptoms. Both are important and necessary, but it does not make a lot of sense to invest substantially in solar or other alternative energy only to throw it out the window through a poorly designed and constructed building envelope. If you focus on reducing your energy needs first, it makes your investment in renewables more valuable because the same amount of generation will cover a greater percentage of your home’s needs.

For instance, by replacing the windows and doors on a client’s 1,200-square-foot townhome and then installing tubular skylights, fluorescent fixtures and a highly reflective roof, we cut the energy needs enough that 1.8 kilowatts of solar PV and a solar water heater sent the townhome into negative energy consumption.

LEED treats geothermal heating and cooling as an energy efficiency measure, rather than renewable energy. I would argue that it is both (extracting renewable geothermal energy out of the ground to heat and storing heat in the ground to cool). Regardless, it is an extremely efficient, reliable and quiet way to heat and cool your home. Geothermal can also augment your hot water needs.

Geothermal's cost

The cost comparison question evokes the big “it depends” qualification. The first costs of geothermal will be more than conventional HVAC systems, but the savings are very substantial. And, as energy prices continue to rise, the investment return gets better and better. 

Geothermal can be closed-loop or open-loop, vertical, horizontal or into a pond or river. Costs will vary depending upon the options available to you due to your site and the labor involved.

  • If you go into the ground (versus a pond or river), the system cost, installation cost, and system efficiency will be governed by your soil type.
  • If your soil easily conducts heat, your costs will be lower because you will need fewer loops in order to achieve the same amount of energy transfer. Sandy soil does not conduct well. Rock conducts well but has high drilling costs. Light, lomy soil and/or wet soil is best. 
  • You should get a professional designer (see the International Ground Source Heat Pump Association) to evaluate your situation and do the calculations.

Some manufacturers have come up with a sleeve system for the loops for unfavorable soil conditions, but I have not had an opportunity to evaluate results. Again, if your house is more energy-efficient, you will need fewer tons of capacity and, therefore, fewer loops drilled (or trenched).

Solar's cost

"Solar" can refer to solar photovoltaics (PV) for electricity generation or solar thermal systems for heat generation.

Costs will depend on your local market and current federal, state, local and utility incentives.

  • In Arizona under current incentives at all levels, solar domestic hot water has about a three-year complete payback. It is a no-brainer even under solely financial considerations.
  • In Arizona, solar PV has a longer payback (10 to 15 years, depending on energy price assumptions).
  • California has perhaps the best opportunities for solar because energy prices are high and incentives are generous.

Costs per watt will depend on your annual days of sunshine, placement and orientation of the panels, angle of the panels (maximum summer power or maximum year-round power), shading issues at different times of the day, ambient temperatures, type of system, manufacturer, size of the system, local panel supply, local labor costs, on-grid or off-grid, battery back-up or not, net metering policies, utility connection requirements, code requirements, and incentives.

If your utility is friendly to solar, you can contact them for info. If not, look up solar installers in your area and talk to two or three of them. If they are using current technology, they can do calculations based on site conditions and incentives that will give you a good estimate of costs and return.

Wind

It sounds like you may have regular wind. Depending on conditions, you may find that wind power makes a better financial investment than solar PV or could augment it.

Consider the portfolio effect of diversifying your energy sources. When the sun is not shining, there is a good chance there could be wind.

My dream system

Good luck with your information-gathering and decision process!

Having gone the energy efficiency and solar route, my dream system would be:

  • Uses geothermal heating and cooling distributed through an in-floor and in-wall radiant system (rather than forced air), powered by solar PV.
  • Hot water would come from a solar water heater storage tank without the heating element, boosted (when necessary) by truly variable, truly tankless point-of-use electric water heaters.

It would require a good green HVAC engineer to design, but the result could be a zero-energy home or one that regularly sends energy back to the grid.

 

For more information:

 

Read Rick Goyette's Q&A "What do you think about geothermal heating and cooling systems?"

Tagged In: solar panels, geothermal

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