Residential Solar Technology Made Simple
Courtesy MOONWORKS Home Improvement
Solar technology is used to convert the sun's energy into useful power. Each day, the sun provides the earth with 15,000 times more energy than we use yet we extract far less than 1% of our energy needs directly from the sun.
Residential solar technology has been around for decades but is largely unknown to the general public. Now with rising energy prices and concern for climate change, solar technology is being deployed at an increasing rate.
Solar Photovoltaic Systems (PV)
Photovoltaic comes from the words photo, meaning "light" and voltaic, meaning "from electricity". A solar photovoltaic (PV) system uses the sun's light energy or photons to produce electrical current. In a typical solar PV panel, light energy from the sun strikes a silicon semi-conductor dislodging electrons causing them to flow in the PV panel which, in essence is the definition of direct current electricity.
As shown in the diagram, a solar photovoltaic system is comprised of a series of silicone solar panels connected to an inverter (our homes use alternating current so the direct current generated by the PV system must be converted to alternating by the inverter) which then supplies the home's lights and appliances.
Most solar PV systems today are “grid connected” meaning that any excess electricity generated by the residential solar PV system is transmitted back to the into the electrical grid and used by someone else (conversely, when the sun is not out, the home draws electricity from the grid).
A residential solar PV system is typically sized to produce most of the home's monthly electricity needs. During peak sunlight hours, the PV system will produce more electricity than the home uses. This excess electricity is transmitted back to the electrical grid causing the utility meter to spin backwards. In the evening and at night, the solar PV system will not be producing electricity and the home will draw its electricity needs from electrical grid (generated by the utility company).
Over the course of the month, the meter will spin backwards when the home is producing more electricity than it is consuming and spin forward when consuming electricity from the grid. The home©owner will pay only for any net electricity used during the month. This is called “net metering”.
Solar thermal technology uses the sun's "therms" or heat energy to generate power. The most common residential application of solar thermal technology is solar hot water where the sun's heat energy is used to heat the home's hot water.
In northern climates where freezing is an issue, solar thermal systems heat water indirectly. A typical system consists of two or three solar panels or collectors as shown in the diagram below.
Unlike solar PV panels, these panels are not made from silicon and instead are made from heat trapping materials. A common solar thermal panel consists of copper tubing which serpentines through the panel and is welded to a copper collector plate surrounded by insulation and encased in heat trapping glass (see below).
A non-toxic antifreeze is circulated through the copper tubing in the panel and is heated by the sun. The heated antifreeze is pumped through a heat exchanger (essentially a glass tube) coiled in a solar water tank where it transfers its warmth into the water. When optimized by an electronic controller, this process can take the water in the solar tank from an ambient temperature of about 50 degrees to the 120 degrees used by the household.
In a typical solar hot water system, the solar tank is used as a “preheat” tank and is placed before the existing hot water tank. When someone in the home uses hot water, the existing hot water tank is drawn down. Rather than being filled by water from the city or well, the tank is filled with preheated solar water reducing or eliminating the need to use the home's conventional water system to heat the water. Over the course of the year, a properly sized and installed solar thermal hot water system can supply up to 75% of the home's hot water needs.
Costs and Benefits of Solar Technology
High upfront costs and long payback periods have hindered the adoption of solar technology in the United States. Additionally, artificially low energy prices (i.e. consumers do not pay the entire cost of keeping oil flowing through the Middle East) and inconsistent government support have also inhibited the growth of solar technology installations.
But these conditions are beginning to change. Technology and manufacturing improvements are lowering the cost and improving the performance of solar technology. The Federal Government has passed a robust incentive program for consumers and producers of renewable energy and these incentives will be in place until 2016. Additionally, a carbon cap and trade system is being considered which will help to allocate the true cost of fossil fuels to their market pricing structure.
Solar Photovoltaic Costs and Benefits
Depending on the size of the home and electrical usage, a typical residential solar PV system is sized to produce between 1.5 kW and 5 kW. The current price per installed kW is approximately $8,000 making the installed cost between $15,000 and $40,000. The federal government will pay 30% of the cost of the system in the form of a tax credit (a direct reduction in federal tax due) and many states offer tax credits as well. Some states, like Massachusetts, offer rebates or direct payments for the installation of a solar PV system.
Even with incentives, at today's electricity prices, the payback period for a solar PV system is still in excess of 10 years. While that may seem long, solar PV systems are designed to last in excess of 25 years and will generate a high return on investment (ROI) over that period. Solar PV systems are very reliable and will generate electricity at prescribed rates shielding the homeowner from escalating electricity costs.
Solar Hot Water Costs and Benefits
As mentioned earlier, a well-engineered solar hot water system is designed to provide 75% of a household's annual hot water needs. A solar hot water system is sized based on the number of people in the household and their respective hot water usage. On average, an individual will use 15-20 gallons of hot water per day. A family of four will use up to 80 gallons per day, which can be generated by a two-panel solar hot water system.
The installed cost of this system (two solar panels, 80 gallon solar hot water tank) ranges between $9,000 and $11,000 depending on the manufacturer and integrator. As with solar PV, the Federal Government will pay 30% or approximately $3,000 in the form of a tax credit and state governments are generally offering tax credits of around 10% bringing the total investment to around $6,000.
The payback period is more difficult to calculate for solar hot water due to the variety of fuels used to provide hot water. In New England, 40% of homes use heating oil to generate hot water. At a current pricing levels of $2.50 - $3.00 per gallon, a solar hot water system general payback in six to eight years making it significantly more attractive than a solar PV system. Solar hot water systems are also designed to last 25 or more years and can generate a return on investment (ROI) in excess of 10%. Additionally, the Appraisal Institute has conducted studies, which show that energy efficiency upgrades increase a home's value making the investment essentially free.
Trends and Future Considerations
Solar photovoltaic technology and its application are changing rapidly. Solar PV panel efficiency has only been around 12-13% meaning that only 1/8th of the sun's light energy is being converted to electricity.
Some new generation panels are now closing in on 20% with designs in the laboratory, which claim to be in excess of 30% efficient. Additionally, pricing for solar panels has been coming down and when combined with increases in panel efficiency and manufacturing techniques, the price per kilowatt should continue to decline.
A newer solar PV technology called thin film PV is beginning to make inroads in the industry. While less efficient than panels, thin film is less expensive and offers more flexibility in its application. Thin film PV can be applied to common building materials like roofing and siding driving the growth in a new product category called building integrated photovoltaic (BIPV). In the years to come, we will begin to see BIVP products on the market, which are increasingly cost competitive with their non-solar counterparts.
Solar thermal technology has been common for several decades. Over this time, manufacturers have improved the efficiency of the systems which now convert over 70% of the sun's heat energy making the technology much more efficient than solar PV technology.
A recent development in residential solar thermal technology is the vacuum tube. The vacuum tubes claim to be more efficient than standard flat plate collectors and can be used in residential space heating applications (although since the sun is weakest in the winter, a well engineered system will still only product 25-30% of the home's heating needs). It is safe to state that solar thermal technology and efficiency will not be changing at the rate of solar PV technology and with faster payback and lower investment costs, solar thermal technology makes most sense to implement today.