You may have a variety of reasons
to invest in solar: to reduce your monthly or overall economic costs, to save
energy (by not using conventional fossil fuels), to reduce harmful
environmental impact now and in the future, and even to promote solar
technology. There may be other reasons. Regardless, you will have to consider
several factors when evaluating such an investment.
The most obvious
consideration is the amount of sunlight available at your site. To determine
how much solar energy is available, you will need to perform (or have someone
else perform) a solar site survey.
Other
considerations will be whether the solar energy is available when you most
need it; the purpose of the application (whether it's to heat your home, your
water, or provide electricity, for example); how well the system integrates
with the structure, finish, aesthetics, and function of your home; the cost of
the system; and cost of the current or alternative energy source. You'll also
want to consider whether the particular application "fits" your
lifestyle, or whether you are willing to make adjustments in your routine
required by the particular application.
The economics of a
solar system depends not only on the cost of the equipment (to install and
operate) and the amount of available free heat, but also on your payback
requirements. You may want a "quick return" on your investment
(e.g., under 10 years) or you may be satisfied with a longer return on the
investment (e.g., 12 to 20 years). Payback is the length of time it takes for
the additional cost of the system to be equaled in energy savings.
Indirect and
Direct Gain
In this design,
the home makes use of both indirect (through the sunspace) and direct gain
(through the second-story south-facing window).
The use of solar in modern
residential applications is not new. People have been successfully applying
modern solar technologies for decades. Some solar applications are
cost-effective, others are not. Some technologies are still undergoing
development while others have reached a developmental plateau. Solar
technology has moved beyond the curious and experimental and has seen
relatively widespread practical application throughout the world. Here are the
most common types of applications available with current technology.
Passive
Solar Space Heating.
The passive solar home is probably the most common application of solar
technology. This system uses the natural principles of heat transfer, air
convection, and thermal mass to store and use solar energy. It requires no
fans or pumps (although there are hybrid systems that use small fans to help
distribute the heat.) It typically incorporates thermal mass into the building
structure or finish materials. It also uses south-facing windows to promote
solar gain; south-facing windows are typically considered a net energy gain
(solar gains exceed heat loss).
Thermal mass is
used to "store" excess solar energy for use at a time when it is
most needed (e.g. at night). Mass absorbs solar energy directly if sunlight
falls on its surface, or absorbs and stores it indirectly as the space
temperature rises and conducts energy into it. Properly designed, you might
expect to get 40% to 60% of your space heat requirements from a passive system
in the Northwest. (See "Back Up Heat.")
There are various
types of passive solar systems. The figure above shows two types: direct gain,
and indirect gain. Each has its proper application.
Passive solar
designs can be very functional while being visually pleasing. Often a simple
reevaluation of standard design and materials can result in a successful
passive solar design, and keep added cost (if any) to a minimum. In a passive
design, materials often double for other purposes. For example, windows are
used for light and view as well as for energy collection. Rock fireplaces,
tile floors, or structural walls, can act as thermal mass.
Active
Space Heating Systems
These systems received a lot of
attention in the mid- to late '70s. They use solar collector panels to collect
energy and fans or pumps to transfer that heat to be used directly for space
heating or to be stored for use at a later time. Collectors use either air or
water to transfer the collected energy. Thermal storage systems often consist
of washed round river rock or water tanks. These systems can provide 30% to
70% of your energy requirements. System costs are generally high because they
require fairly large collector areas and storage systems.
Active
Domestic Hot Water (DHW) Systems
These systems use solar collectors, a pump to circulate water, and store heat
in either a separate storage tank (e.g. a "preheater") or the normal
DHW tank. They have freeze protection modes to protect the collectors from
freezing during cold nights. There are several types of freeze protection
methods including: glycol water mixture using a heat exchanger, draindown
systems, and drainback systems. Each has its advantages and disadvantages.
Figure 2 illustrates an active hot water system.
A properly sized
system can provide 60% to 70% of your domestic hot water needs. Some active
DHW systems have had problems associated with the controls that are to protect
the system from freezing and optimize its efficiency. If the controls don't
work properly, the system will not work or may work inefficiently.
Active Hot Water
System
In this active hot
water system, water is stored in a pre-heating tank.
Passive
DHW Systems
These systems use the natural
circulation characteristics of thermosyphoning (the phenomenon of warm water
rising and cool water falling) to circulate water through the collector into
the storage tank. Most systems require the storage tank to be above the
collector to use this principle. One system uses the same principle as a
percolator coffee pot to circulate water through a collector above the storage
tank.
Solar
Swimming Pool and Spa Heaters
These are active water heating
applications. They require relatively low-temperature heat, which make them
good candidates for this technology. Pools may only require a temperature of
around 90°F and spas around 110°F while DHW and active space heating systems
require temperatures of 120°F and higher. Pool applications can often provide
a higher percentage of the energy requirements than domestic hot water and
space applications. Swimming pool applications make economic sense, even if
the use of the pool is seasonal. Spa applications work well, because even
though their temperature requirements are higher than swimming pools, the
volume of water to be heated is much lower.
Photovoltaic
(PV) Systems
These systems use solar cells to collect and convert sunlight directly into
electrical energy. A storage system (e.g. lead acid batteries) is needed to
provide power at night. This is the same technology that is used in the space
program to provide power for satellites. It is often used in marine
applications, such as on boats or buoys, and you may see them on highways
supplying remote emergency phones. Residential PV systems have seen more
widespread application in the last decade. (Recent estimates indicate that
there are now 30,000 PV homes in the U.S.) This is partly due to research and
development efforts to reduce the cost of solar cell manufacturing. This
technology promises continued cost reductions in the future. This combined
with cost increases of electricity from conventional sources make PV systems
more and more viable. PV technology does not require direct sunlight to
function (i.e. it can be cloudy, although cloudiness does reduce the energy
output).
What are the best
residential PV applications? Most residential applications are limited
(economically) to rural or remote applications where the utility grid is more
than a mile away. To be cost-effective, a typical PV application depends upon
conservation, and may require other modifications. For example, alternative
appliances may have to be used, and some other system will have to be used to
supply space heat.
Solar
Ovens
Solar "cookers" use a series of mirrors or reflective panels
arranged around a "box oven." Drawbacks to practical use includes
the fact that solar ovens are generally not easily applied or adapted to the
average kitchen. Another is that solar ovens require bright, direct sunlight
and the mirrors must be lined up with the sun. Figure 3 shows how a solar oven
works.
First:
Conserve!
Solar applications in homes should not be a substitute for conservation
measures. By reducing energy needs, energy conservation strategies will reduce
the size and cost of the solar system required. Whether you choose a solar or
conventional energy system, it's best to apply conservation measures first.
Back
Up Heat
In the Northwest, there can be several weeks in the winter where substantial
sunshine is lacking. Since the availability of solar energy is not absolutely
dependable, one common trait of most solar systems is the use of a
"back-up" heating system. Conventional heating methods are typically
used. This should be taken into consideration when estimating the cost of the
solar system.
State
Standards
Washington State has an Energy Code (WSEC) that provides a mandatory standard
for energy efficient new construction. The Code stipulates minimum insulation
levels for ceilings, walls, floors, windows, and doors for new buildings and
additions. Some standards relate specifically to solar energy gains and the
use of thermal mass to store such gains. In addition, the Ventilation Indoor
Air Quality (VIAQ) Code provides standards to insure good indoor air quality.
These include specific requirements to prepare for potential radon mitigation.
The codes are enforced by the building department in your local community. If
you are building a new home, or making an addition to an existing home, you
must meet the energy code. The Code also addresses remodelling projects,
though requirements may be less stringent. Still, the energy code is a good
benchmark to shoot for. Homes built to the energy code are more quiet,
comfortable, and less costly to operate.
