Hopper Design, LLC


Toward Healthy and Sustainable Homes

Energy - Production

After taking substantial steps towards conservation it is worthwhile to seriously consider producing energy on-site. In decreasing priority one should think about producing:

  1. Solar domestic hot water,
  2. Solar space heating and
  3. Solar electricity.

For any incoming unit of solar energy a solar thermal system will produce about four times as much energy as that produced by an equal sized solar electric system.

You can save up to an additional 30% of your energy needs by adding on-site renewable energy production compared to a conventional home. Together, conservation and on-site energy production can reduce your nonrenewable energy needs a total of 80% compared to a conventional home.1 It is even possible to approach zero net energy with the systems described here.

In order of importance:

1. Solar Thermal Domestic Hot Water

With a solar thermal domestic hot water (DHW) system we capture solar energy in thermal solar collectors to make hot water on-site for domestic use. This is a thoroughly tested technology that is simple, robust, has long service life and requires little maintenance over its life. A large, well-insulated storage tank is used to hold the hot water. This is the first active system to consider because of year-long economic benefits. A practical installation can save 75% or more of your domestic hot water needs, requiring little more than a high efficiency instant-on gas heater for backup. Low wattage pumps and low voltage controls are required to automate operation of the system.

To learn more about solar domestic hot water see DOE: Solar Hot Water and Space Heating & Cooling (pdf).

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2. Solar Thermal Space Heating

Capturing thermal solar energy for space heating is a technology just like solar domestic hot water described above and simply requires a greater area of collection along with larger storage. This system is especially appropriate for radiant floor designs since these systems can heat a home with water at a relatively low temperature, improving overall system efficiency. A practical installation can save 50% or more of your space heating needs, requiring little more than a high efficiency instant-on gas heater for backup. Low wattage pumps and low voltage controls are required to automate operation of the system.

A practical stand-alone solar thermal space heating system is radiant heating in an insulated sand bed thermal mass set below a concrete floor. Any backup heating system should be a totally separate system. This approach insures serviceability of each system.

In a second floor application, an important benefit of a gypsum concrete encased radiant floor system is that it provides thermal capacitance, and is useful in storing passive solar energy, see Conservation.

To learn more about solar thermal space heating see DOE: Solar Hot Water and Space Heating & Cooling (pdf).

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3. Solar Electricity with Photovoltaics

Here we capture solar energy with photovoltaics (PVs) for use as electricity. PVs produce DC current which must be converted to AC (with an inverter) for household use. This energy can be used to power your house directly or feed the grid if you are not using much power. This is accomplished by using grid-tie systems, i.e., by using the grid as a battery. While a net meter can wind backwards on occasion, most installations are in the 3 1/2 to 5 kilowatt range, which typically reduce electric energy needs by about 1/3 or 1/2 of total usage. Actual net energy usage is dependent on energy needs and lifestyle.

To determine how much electrical energy you can expect to receive at your location, see PVWATTS (Version 1).

To learn how PVs produce electricity from sunlight, see PV cells (pdf).

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Discussion: As mentioned in Conservation, you can save a total of about 50% of your energy needs compared to a conventional home simply by conserving energy. Each of the on-site production strategies above can reduce your energy needs by about 10%, so that assuming you implement items 1, 2, and 3 above you can save about 30% with production techniques. Together, conservation and production can reduce your energy needs by 80% compared to a conventional home.

You may choose to continue to higher savings and even to approach zero net energy usage. This can be done, for example, by installing a larger PV array, or even a windmill if your site is suited for wind energy.

Zero net energy means your annual energy usage and production ("deposits" and "withdrawals") are equal.1 To realize a zero net energy home it will be essential to include all the steps for energy conservation and production that we describe. In addition you must go further in terms of on-site energy production by, for example, installing a larger PV array, or even a windmill if your site is suited for wind energy. Zero net energy is most easily achieved by tying your home to the grid. By implementing these strategies you will substantially conserve limited natural resources and reduce your carbon footprint further.

See also: Tax credits from the Energy Policy Act of 2005. The IRS provides information on tax credits for individuals who make energy efficient home improvements and for those who add active solar systems, and Database of State Incentives for Renewable Energy (DSIRE) State and federal incentives for energy efficiency.