Solar Powered Heating from the Sun: With Electricity or Hot Water

Video: Paul Ledman's net-zero apartment building in Portland, Maine which uses 100% solar for heating and cooling and will never need fossil fuel inputs.

ReVision Energy specializes in designing renewable energy heating systems for high-efficiency homes where the heating load is low (relative to the needs of most homes in New England).

Typically the relative efficiency of houses falls into one of three broad categories:

Air Source Heat Pumps

Another electric heating option are air source heat pumps. By extracting heat from the outside air and using it in a reverse refrigeration cycle, the heat pump can generate 2-3 units of heat for every unit of electricity it consumes. This is like having an resistive electric heater operating at 200-300% efficiency and that ratio is commonly referred to as the coefficient of performance (COP) of 2 -3.

Like resistive electric, the best way to make an air source heat pump "green" is to use grid-tied solar electricity to offset the energy use. We size the system based on yearly load, so your PV system will bank excess energy generated in the summertime, so you can use it in the winter.

Historically air source heat pumps only worked in moderate heating climates like Florida or the Mid Atlantic, but thanks to new refrigeration technology, the latest generation of air source heat pumps work down to temperatures as low as -6 degrees F at exceptionally high COP's.

There are two types of air source heat pumps that we install:

  • Air/air heat pumps - Heat the indoor air directly.
  • Air/Water heat pumps - Heat water which is subsequently used in a hydronic distribution system.

Air to Water Heat Pump - The Daikin Altherma

The air to water heat pump is like a boiler in that it heats water, but it does so without burning anything. Instead it uses a refrigeration cycle to pull heat out of the outdoor air and uses that to heat the water which is then circulated through the floor and radiators.

The Daikin Altherma is the best of these units and comes in a variety of sizes to meet a home's load (more info on Daikin's website).

Altherma heat pump diagram

Because the Daikin doesn't burn fuel for heat, the only operating cost of the heat pump is the electricity to run the pumps and the refrigeration cycle.

Our good friends at Northeast Radiant Technology (NRT) recently installed an Altherma heat pump for their office building; case study here: Heat Pump and Rooftop Solar Puts Northeast Radiant on Path to Net Zero.

Air to Air (Ductless Mini Split) Heat Pumps

Jojoba Company - Waldoboro, Maine
The Butler Barn, a facility operated by the Jojoba Company, uses PV to power the office and an air source heat pump for heating.
This heating system heats air, not water, so a home doesn't need a hydronic distribution system at all. Instead, a number of inside units blow warm (or cool) air to condition the space as necessary.

Ductless mini splits, like air to water heat pumps, use heat in the outdoor air to heat a home's living space, but instead of doing it through the circulation of warm water, they heat the air via wall mounted units: http://www.mitsubishicomfort.com/

For a 2,500 square foot building with a fairly open concept we generally install 2 of these heat pumps (one per floor) and then count on natural convection and the air exchange/heat recovery system to equalize the heat throughout the space. If needed, we will install simple supplemental resistive electric heat in the spaces that end up not heating as well as needed with the heat pumps alone (often this is suggested for at least the bathrooms).

Once nice advantage of this type of system is that you automatically get an efficient air conditioner as part of your heating system.

What About Ground-Source Heat Pumps? (Geothermal)

Ground-source heat pumps (AKA geothermal) have generated a lot of buzz in recent years, but we find that for homes with a low enough electric load (<50,000btu/hr), we can design a simpler and more reliable system with a free fuel source to run the system for the next 30-50yrs - the sun!

In these designs, we will install resistive electric base board, electric boiler or an air source heatpump combined with a large grid-tied solar electric (PV) array.

Why use grid-tied solar for heating?

  • Affordability - Solar electric systems are more affordable than ever before, with great incentives and historic low pricing on photovoltaic panels. By choosing solar electric, you pre buy your fuel for the next 30-50yrs and insure yourself against future energy price increases.
  • Reliability - Unlike geothermal, solar electric panels are maintenance-free, warrantied for 25 years, and are expected to last 30+ years. We back all of our systems with 24/7 emergency service.
  • Convenience - Your PV system will be generating electric credits all summer long which can be used to heat your home in the winter. One ReVision employee put it this way - you can split and stack wood, OR you can go to the beach and let the sun do the work for you!

Solar Space Heating

Round Pond, Maine - Solar Hot Water Space Heating
This home in Round Pond, Maine uses ground mounted 240 evacuated tube solar hot water collectors.
This system produces more than 65,000,000 BTUs of clean, renewable heat energy per year for domestic hot water and space heating.

We're often asked whether we can integrate our solar hot water systems with conventional boilers to produce energy for space heating. Solar space heating is doable, but has a few special requirements:

  • Solar space heating only makes sense in higher performing homes
  • Solar space heating requires low temperature distributed (e.g. radiant floor) heating
  • Solar space heating requires excellent southern exposure

For retrofits, solar hot water is rarely a viable option. Solar is not great at heating water to high temperatures for baseboard heat, and even homes with existing radiant heat are usually not insulated enough to make the investment in a solar space heating system worthwhile (however, domestic solar hot water remains an excellent choice).

See our Adobe PDF Solar Space Heating (PDF) document for more details.

In homes where solar space heating is viable, it is still a supplementary form of heat - with the added benefit that it will also produce over 90% of the home's domestic hot water, year round!

Since solar availability is at its lowest when heating needs are at their highest, solar space heating requires a form of backup heat. Solar space heating can work well when paired with a high efficiency gas, propane, or wood boiler.

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Cape Elizabeth, Maine - Solar Hot Water

We love getting hot water from the sun. The installation looks fine on our old farm house.

This summer we had 13 people living, showering, cooking and doing laundry here and the furnace didn't go on even once. Hooray!

Red and Louise Sullivan
Two Lights Rd., Cape Elizabeth

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