Solar energy is
a subject both broad and deep. In this section, you will be introduced
to
the basics of
solar electric
and
solar hot water systems,
as well as some important information about
energy
efficiency and conservation.
Also included is a section discussing the financial considerations of
solar energy. At the bottom of this page are a list of
resources
that you might find
useful in your exploration of solar energy.
Solar Electric
-
Solar electric systems convert the energy embodied in sunlight into
electricity. Solar electric systems include solar
modules
(solar panels) that convert the energy in sunlight to direct current (DC) electricity,
power conversion
equipment called inverters
that invert
DC electricity to alternating current (AC) electricity
as well as safety disconnects and various metering equipment.
Some solar electric systems include batteries
to provide
power to household appliances during power outages or to power off-grid
homes. Some systems include backup
generators to supplement solar generation to keep the
batteries up. There are three main classifications of solar
electric systems that can be installed in your home or business: direct grid-tie, grid-tie
with battery backup and off-grid.
-
Direct Grid-Tie
Direct
grid-tie systems operate in conjunction with the utility grid to
achieve optimal efficiency. When the sun is shining and your
solar electric system is producing electricity, the first priority is
to power your household appliances. If a surplus of
electricity is being produced, the solar inverter feeds any excess
electricity back to the utility grid, making your utility meter
spin backwards.
This builds up a credit with your electricity company,
reducing your bill, as well as increasing the amount of green energy on
the utility grid. If not enough electricity is being produced
by your solar electric system to power your household appliances, power
is brought in from the utility grid to fill in the gap. All
of this happens automatically without any need for intervention from
you.
Direct grid-tie systems do not include batteries. This increases system
efficiency as well as reducing equipment and installation costs.
These systems use battery-less
inverters that are not compatible with battery banks.
Therefore, direct grid-tie systems cannot provide power to
your household appliances during power outages. Direct
grid-tie systems can be retrofitted with batteries for backup
capability.
-
Grid-Tie with
Battery Backup
-
Grid-tie
with battery backup systems offer the flexibility of being connected to
the grid with the security of having backup power during a power
outage. These systems feed excess electricity back to the
grid in the same way that a direct grid-tie system does, as well as
providing power to household appliances during power failures.
Grid-tie with battery backup systems use battery
based inverters and a battery
bank to store electricity for later use. Because
batteries are included in the system, a solar
charge controller is
needed to optimally charge the battery bank and prevent over charging.
A backup generator can be included as well for continued
power during prolonged power outages. It is usually not
practical to power your entire house during a power outage, so a
separate electrical panel, called the critical loads panel, powers
critical appliances such as a well pump, refrigerator, heating system,
lighting and any other appliances which would increase your comfort.
-
Off-Grid
Off-grid solar electric systems can provide power equal to or
surpassing quality of grid service to a remote home location, often at
a fraction of the cost.
Larger battery banks
are used in conjunction with optional backup
generators to provide power throughout the year.
Off-grid systems use battery
based inverters as well as solar
charge controllers.
-
Solar Electric System Components
-
Solar Modules (Solar Panels) are the components that
convert the energy embodied in
sunlight into electricity. They consist of solar cells wired
together and encapsulated in an aluminum frame and a glass top.
Solar modules are rated in Watts.
If a module is rated at 200 Watts, this module will produce
200 Watts under certain conditions. (Those certain conditions
are a loose approximation of real world conditions. Usually
the actual output of a solar module is 5-15% less than the
manufacturer's rating.) As an example, you could power two
100 W incandescent light bulbs or six 33 W compact fluorescent light
bulbs (which would produce three times as much light) off of one 200 W
solar module.
Modules are wired together to form an array of
solar modules. The physical size of a 1000 W array (five 200
W solar modules) is roughly 75-100 square feet. A typical
Vermont
home would need a 4500 W array to provide half of its electrical needs,
while an energy efficient Vermont home would need a 4500 W array or
smaller to provide all of its electrical needs.
Solar arrays can be mounted on the roof of a house or
anywhere that gets good sunshine through out the day. It is
imperative that the array be in the shade as little as possible as this
will greatly reduce its output.
- Inverters are the
components that convert DC electricity that is produced by the solar
modules to AC electricity that can be used by your household appliances
and also be fed back to the utility grid. Battery-less
inverters simply feed solar electricity into your household appliances
and back to the grid if there is an excess. Battery based
inverters perform this function as well as use a battery bank to
provide back up power during a power outage. Inverters are
also rated in Watts. This rating
signifies the amount of electricity the inverter can invert (convert
from DC to AC) continuously. If an inverter is rated at 3000
Watts that means it can handle a 3000 W solar array for grid-tie
systems or power 3000 W worth of household appliances for off-grid or
backup power situations.
-
Batteries
are the components that store electricity for later use in off-grid or
grid-tie with battery backup systems. Batteries are wired
together to match the inverter and to provide sufficient storage
capacity. This is called a battery bank. Battery
banks typically store anywhere from 4-16 Kilowatt Hours or more for use by
your household appliances. Most batteries need periodic
maintenance totaling several hours per year to keep them in good
condition. Maintenance free batteries can also be used,
although there is an increase in cost.
-
Solar Charge Controllers are the components that charge
the battery bank from a solar array. They use specific
charging algorithms to increase battery life and prevent overcharging
of the battery bank.
Solar
Hot Water
-
Solar hot water systems use the energy embodied in sunlight to heat
water directly for domestic hot water, space heating or other hot water
needs. This is achieved by the use of solar
hot water
collectors connected to one or more hot water storage tanks
by piping.
A pump is used to circulate a heat
transfer fluid through the
collectors, allowing it to heat up, and return the heated fluid back to
the storage tank. A heat
exchanger at or in the storage tank
is used to transfer the heat in the transfer fluid to the water in the
storage tank. The cooled heat transfer fluid is then returned
to the collectors and the process starts all over again.

Solar
hot water systems usually include two tanks, one to store the solar
heated water and one to provide backup water heating during cloudy or
cold periods. Most often, your existing hot water tank will
function as the backup tank. The output of the solar storage
tank is piped to the input of the backup tank, so any water entering
the backup tank has been preheated by solar energy, giving you a
significant energy savings. For homes with low hot water
usage, a single tank can function as both the solar storage tank and
the backup tank with good results.
-
Solar Hot Water System Components
- Solar Hot Water Collectors
are the components that collect the energy embodied in sunlight by
heating the water inside of them. They consist of a metal frame with
copper piping inside and a glass cover on top. As heat transfer fluid
circulates through the collector, it absorbs heat from the sunlight.
Hot water collectors can be connected together like solar electric
modules to achieve more water heating. Each collector is
generally 4' x 8', but some are larger or smaller. Unlike
solar electric modules, hot water collectors must be placed as close as
possible to the solar storage and backup tanks, preferably on the roof
of the house. This allows for shorter pipe runs so that less
heat will be lost during transport of the heat transfer fluid.
- Hot Water Storage Tanks
are the components that store hot water for later use. Some, like the
hot water tank in your home have a heat source to heat the water for
storage. Solar storage tanks do not usually have a heat source because
they are storing hot water that has been heated by the solar hot water
collectors. Solar storage tanks are usually larger than normal hot
water tanks because that allows you to store solar heated hot water for
cloudy periods. The more storage you have, the more you are going to be
using water that has been heated by the sun instead of by electricity
or gas. Some storage tanks have a heat
exchanger integral to the tank, while others need an external
heat exchanger.
- Heat
Transfer Fluid is the fluid that absorbs heat
in the solar hot water collectors and transfers that heat to the water
in the solar storage tank
by means of a heat exchanger.
Heat transfer fluid is most often a mixture of non-toxic propylene
glycol (a type of anti-freeze) and water. The addition of the glycol
prevents the heat transfer fluid from freezing during the winter.
- Heat
Exchangers are the components that transfer
heat from the heat transfer fluid
to the water in the solar
storage tank. They are necessary to prevent the
mixing of the glycol in the heat transfer fluid and the potable water
in your solar storage tank. Heat exchanger designs vary.
Some common types are tube-in-shell and flat plate heat
exchangers.
Energy
Efficiency & Conservation
-
Energy efficiency and conservation are the easiest and most cost
effective ways to reduce your energy cost and pollution related to
energy usage. Something as easy as changing the most
frequently used light bulbs in your home can reduce your electric bill
by up to 25%. Energy efficiency and conservation are also
important when utilizing solar energy systems. Time and money
spent on efficiency and conservation will drastically reduce the size
and cost of a solar energy system needed to achieve the same reduction
in household energy costs. Many households can reduce their
total energy bills by as much as 50% by implementing or retrofitting
their home with energy efficient building products and practices.
Simply becoming aware of your energy usage by calculating the
cost of certain appliances or installing a household electricity
monitor can be a big step to reducing your energy costs. The Resources section below has many
good websites for further information about this important topic.
Financial
Information
-
Money can be one of the biggest stumbling blocks to owning a solar
energy system. Fortunately, our state and federal governments have
deemed solar energy technology worthy of investment and have made money
available to consumers in the form of rebates, tax incentives and
loans.
- State incentives are primarily twofold. First, equipment
for renewable energy systems are sales tax exempt. Second,
the Vermont
Energy Investment Corporation funds a renewable energy rebate
program administered by the Renewable Energy Resource Center.
This program provides a rebate for solar electric, solar hot water and
small wind systems that are installed by Vermont Solar or Wind
Partners. Solar
and Wind Partners are companies that have met certain criteria and
have been approved by Renewable Energy Vermont to
reserve state rebate funds for their customers. Bright Earth
Solar is a Solar Partner and is approved for solar electric and solar
hot water systems. Currently the rebate is $1.75 per watt of
installed DC system capacity for solar electric and $1.75 per 100 Btu
per day of installed system capacity for individuals and businesses.
For example, if you purchased a solar electric system with 1500 Watts
of solar modules, you would receive a $2,625 rebate. For more
information about the Vermont Solar & Small Wind Incentive
program visit the Renewable Energy Resource Center
website.
- Federal incentives for renewable energy systems focus on a
personal or corporate tax credit. A tax credit is a dollar for dollar
reduction in taxes due as opposed to a deduction which reduces the
income on which you are taxed. Currently, the personal tax credit is
30% of the system cost or $2,000, whichever is lesser on solar electric
and solar hot water systems. The corporate tax credit is 30% of the
system cost for solar electric and solar hot water systems. For more
information about the federal tax credit or about other incentives
visit the Database
of State Incentives for Renewables & Efficiency.
Resources