There are two common solar technologies: solar photovoltaic (PV) and solar water heating.
Solar Photovoltaic (PV) Systems
Photovoltaic systems are made up of photovoltaic cells, AC/DC inverters, a racking system, and sometimes, a battery for energy storage. PV cells convert the energy from the sun into electricity and are arranged together to form solar panels. The inverters then convert the electricity generated by the panels from direct current (DC) to alternating current (AC), in order to power your house.
Residential solar systems are almost always connected to the power grid. When the sun is shining and you are not using too much power in your house, the excess energy generated by your panels is sent to the grid. When there is not enough sun to meet your energy demand, you take energy from the grid. By connecting to the grid you are ensured that you’ll have power to hour home at night or on cloudy days.
To learn more about how the electricity produced by your solar system gets credited to your electricity bill, read our Net Metering page.
PV cells – PV cells are interconnected to create modules and panels. The most frequent panels for residential installations are crystalline silicon panels. The number and size of panels on your roof will depend on your electricity usage and, more importantly, the size of your roof.
Inverters – There are two types of inverters: central inverters and micro-inverters. Central inverters receive all the energy generated by the panels and convert it to AC, while micro-inverters are connected to each panel, converting the electricity from DC to AC immediately.
Central inverters are usually located at ground level near the main electrical service panel, and a wire that connects your panels run from the roof to the inverter. Central inverters optimize energy conversion for the entire system, but if one panel is shaded it can bring down the performance of the rest of the system.
Micro-inverters, on the other hand, work individually with each panel and if one pane is shaded it does not impact the production of the other panels. Micro-inverters are also optimized for the production of each panel and can increase power production by 5 to 25% compared to central inverters. Some micro-inverters offer online monitoring that allows you to see how your panels are performing. Read more about inverters.
Racking system – How your solar system is mounted on your roof will depend on several factors. There are a number of different racking system options, including ballasted systems, beams attached to the walls, and others that connect to struts on your roof. Installers typically decide on the best racking system to use based on how your roof is structured.
Operations and Maintenance
Solar systems have no moving parts, so they do not need much maintenance. Occasionally squirrels will chew on the wires that run from the panels to your electric box, but this is easy to repair if it does happen. Because Maryland gets a fair amount of rain each year, you do not need to clean or wash your panels. In the event of snow, we recommend you do not try and remove snow from your roof. Because the panels are dark, they absorb heat and will melt the snow from your roof quickly.
Solar Water Heating Systems
Solar water heating (SWH) systems capture the sun’s heat energy and use it to heat water or other liquids. The system collects the sun’s energy using a solar collector. That energy is then transferred into a water storage tank, similar to those used by a conventional water heater. Systems have a backup heater to ensure that you will always have hot water, even if the sun isn’t shining.
They payback time for solar water heating systems depends on whether you heat water with gas or electricity.
There are two common types of solar collectors used for residential and small commercial applications: flat plate and evacuated tube collectors.
Flat plate collectors – Flat plate collectors are the most common and cost effective type of collector for domestic solar hot water systems. These collectors are insulated, weatherproof boxes that hold an absorber plate. The sun heats this plate and transfers the heat to a heat transfer fluid flowing through tubes in or near the plate. Flat plate collectors can typically produce temperatures up to 180 degrees F and in sunny, warm conditions can have efficiencies in the 65% to 78% range, but lose effectiveness when it gets extremely cold or cloudy. Flat plate collectors are the less expensive type of panel.
Evacuated tube collectors – Evacuated tube collectors are a parallel series of rows of transparent glass tubes that surround rows of absorbers. The air in the tubes is then removed (evacuated) to form a vacuum. This reduces heat loss and increases efficiency and, as a result, evacuated tube collectors can reach temperatures of up to about 300ºF. Evacuated tube collectors will typically have top efficiencies in the 40% to 55% range, but retain their effectiveness better in extremely cold weather. This type of collector is more expensive than flat plate collectors, but is also more efficient.
In addition to the collectors, SWH systems require a hot water storage tank(s). In a two-tank system, the water is preheated in the solar tank using heat exchangers before entering the original hot water heater. In a one-tank system, the solar heat exchanger is built into a single tank. A circulating pump circulates the heat transfer fluid from the collectors on the roof via insulated pipes (typically copper) and additional plumbing connectors. Temperature sensors are usually installed in the collector and water tank. These are connected to a controller, which turns on the pump when fluid in the solar collector is approximately 15°F hotter than the water in the tank. Systems may also contain an expansion tank, a safety measure that can relieve excess pressure in the system if it gets too hot. In addition, some controllers can measure the amount of energy being produced by the system.
Operations and Maintenance
Once installed, solar water heating systems require minimal maintenance. Over the 30+ year lifetime of a solar hot water installation it is estimated that the homeowner will need to replace the water pump after 10-15 years, at a cost of $100-$300. Additionally, excess heat can cause the glycol mixture in indirect systems to deteriorate and corrode system components. Homeowners should have their glycol checked and changed (if necessary) every 3-5 years. This typically costs about $100-$200 per visit.
*Excerpt from “Guidebook for Solar Water Heating Projects,” December 2011.