How Solar Thermal Energy Works

Solar water heaters, also called solar domestic hot water systems, can be a cost-effective way to generate hot water and space heating for your home. They can be used in any climate, and the fuel they use, sunshine, is free.

How They Work

Solar water heating systems include storage tanks and solar collectors. There are two types of solar water heating systems: active, which have circulating pumps and controls, and passive, which don't.

Most solar water heaters require a well insulated storage tank. Solar storage tanks have an additional outlet and inlet connected to and from the collector. In two tank systems, the solar water heater preheats water before it enters the conventional water heater. In one tank systems, the back-up heater is combined with the solar storage in one tank.

Three types of solar collectors are used for residential applications:

Flat plate collector

Glazed flat plate collectors are insulated, weather proof boxes that contain an absorber plate under one or more glass or plastic (polymer) covers. The absorber plate is dark, most often painted with a solar selective coating, also known as "Black Chrome." Solar selective coatings absorb both visible light and infrared, making them highly effective at converting the sun's energy to heat. Unglazed flat plate collectors, typically used for solar pool heating, have a dark absorber plate, made of metal or polymer, without a cover or enclosure.

Integral collector storage systems

Also known as ICS or batch systems, they feature one or more black tanks or tubes in an insulated, glazed box. Cold water first passes through the solar collector, which preheats the water. The water then continues on to the conventional backup water heater, providing a reliable source of hot water. They should be installed only in mild freeze climates because the outdoor pipes could freeze in severe, cold weather.

Evacuated tube solar collectors

They feature parallel rows of transparent glass tubes. Each tube contains a glass outer tube and metal absorber tube attached to a fin. The fin's coating absorbs solar energy but inhibits radiative heat loss. These collectors produce higher temperatures than flat plate collectors. They are used more frequently for in commercial applications.

There are two types of active solar water heating systems:

Direct circulation systems

Pumps circulate household water through the collectors and into the home. They work well in climates where it rarely freezes.

Indirect circulation systems

Pumps circulate a non-freezing, heat transfer fluid through the collectors and a heat exchanger. This heats the water that then flows into the home. They are popular in climates prone to freezing temperatures.

Drain back systems are popular in freezing climates. A drain back system includes a reservoir or tank that holds the heat transfer fluid, most often distilled or demineralized water. When the system is active, a pump moves the fluid up through the collectors then through a heat exchanger, where the heat is transfered to the domestic hot water system. When the system turns off, all of the heat transfer fluid drains out of the collectors back into the reservoir, thus providing freeze protection to the collectors

Illustration of an active, closed loop drain back solar water heater. Two large, flat panels called flat plate collectors are connected to a tank called a drain back tank by two pipes. One of these pipes comes from the bottom of the drain back tank and runs through a centrifugal pump. The pump moves the Heat Transfer Fluid (HTF) up through a flow meter then up to the flat plate collectors mounted on the roof. The HTF is heated by the collectors and returns to the drain back tank via the second pipe. In the bottom of the drain back tank, there is a coil called a heat exchanger. Water from a solar heat storage tank is pumped through the heat exchanger removing the energy from the HTF in the solar loop. These are two separate loops, one non potable loop to the solar collectors and one potable water loop through the solar storage tank. The back up hot water tank has a conventionally fueled back up heating system, usually electric or gas, in case there are prolonged cloudy periods when the solar collectors are not producing heat.

Solar water heating systems almost always require a backup system for cloudy days and times of increased demand. Conventional storage water heaters usually provide backup and may already be part of the solar system package. A backup system may also be part of the solar collector, such as rooftop tanks with thermosyphon systems. Since an integral collector storage system already stores hot water in addition to collecting solar heat, it may be packaged with a demand (tankless or instantaneous) water heater for backup.

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How Photovoltaics work

Solar cells, also called photovoltaics (PV) by solar cell scientists, convert sunlight directly into electricity. Solar cells are often used to power calculators and watches. They are made of semiconducting materials similar to those used in computer chips. When sunlight is absorbed by these materials, the solar energy knocks electrons loose from their atoms, allowing the electrons to flow through the material to produce electricity. This process of converting light (photons) to electricity (voltage) is called the photovoltaic (PV) effect.

Solar cells are typically combined into modules that hold about 40 cells; about 10 of these modules are mounted in PV arrays that can measure up to several meters on a side. These flat plate PV arrays can be mounted at a fixed angle facing south, or they can be mounted on a tracking device that follows the sun, allowing them to capture the most sunlight over the course of a day. About 10 to 20 PV arrays can provide enough power for a household; for large electric utility or industrial applications, hundreds of arrays can be interconnected to form a single, large PV system.

A typical commercial solar cell has an efficiency of 15 percent. That means that out of all of the energy striking the surface of the solar panel, about 15 percent is converted to electricity. The rest either passes through the panel or is converted to heat. Heat effects the efficiency of solar panels inversely. The hotter the panel gets, the less efficient it is, thus in cooler climates, like the Hudson Valley, solar cells usually operate more efficiently than in warmer areas like Florida. Age also effects efficiency, the older a panel is, the less efficient they become. Photovoltaic panels have a useful life of 25 years where the panel is producing 80 percent of its rated value.

Thin film solar cells use layers of semiconductor materials only a few micrometers thick. Thin film technology has made it possible for solar cells to now double as rooftop shingles, roof tiles, building facades, or the glazing for skylights or atria. The solar cell version of items such as shingles offer the same protection and durability as ordinary asphalt shingles. Thin film is less efficient than other types of solar panels, however, they generally cost less too.

Solar systems can either be independent or grid connected. Both types of systems use an inverter to convert the DC voltage from the solar cell to AC voltages to be used by standard appliances.

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Grid Connected systems

Grid connected solar systems use an inverter that synchronizes with the utility power. These systems do not generally require batteries, although batteries can be used to provide back up power if the utility power goes out. Grid connected systems are generally more simple to design and install, but they also have the most regulations regarding installation. The National Electrical Code section 690 deals with grid connected PV systems. Most states have Net Metering agreements in place with utility companies, which also have strict regulations regarding connection of power inverters to the electrical grid. Inverters used with Grid connected systems must meet UL 1741 specifications to provide anti islanding protection. Finally, many states have rebate programs in place to off set cost associated with installation of grid connected PV systems. These programs also have many rules and regulations regarding installation and use.

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Non grid connected systems

Independent systems are most often used in remote locations where utility power is not available. These systems are more complex, more expensive and require careful design to meet all of the electrical load criteria.

There are several design considerations that are not factored in in an on grid system.

1. The system should be sized for greater than 100 percent of the electrical usage, unless a back up generator is planned for the site. The reason for this is to compensate for cloudy days.
2. The battery bank should have enough Amp hours (Ah) to run for at least two to three days continuously without sunlight.
3. An Off grid solar system can also augmented with a wind generator. These arrangements usually work well because when the sun is not shining, the wind is usually blowing.
4. Off grid systems or grid tie systems with battery back up require more maintenance and batteries can be an explosion hazard.

For off grid sizing, all of the electrical loads need to be added together to estimate how much storage is needed. This take into account how often each load is used every day, and what loads are going to running at the same time. When calculating load, use the heaviest load rating. Well pump motors are a good example of equipment that has a high starting current, but a normal running current. A 1 horsepower pump motor should draw around 1,194 watts when running (60% motor efficiency), but will draw almost 4,000 watts when starting.

This will looks something like this:

• Step 1: Load X quantity X volts X amps = watts
• Step 2: Watts X hours per day X days per week /7 days = Average watt hours

The following work sheets can make it easier to calculate loads, batteries, inverters and photovoltaic arrays:
Note: there are down loadable .pdf files of these work sheets in the down load section below

Stand Alone Electric Load Individual loads qty X Volts Amps = Watts Watts DC Hrs per day Days per week /7 days= AC watt hours DC watt hours                                                                                                                                                                                                                                                                                                                                     Total     Total connected AC watts___________ AC average load per day _______________ Total connected DC watts___________ DC average load per day _______________

Then, for a safety factor, multiply the total connected watts by 1.25. The inverter (if the loads are using AC power instead of DC power) should be sized to handle at least 120 percent of the AC load. This will give the system some room for expansion and for starting heavy loads.

The battery bank is sized to the load and the number of autonomous days required. The sizing looks something like this:

Battery Sizing AC Average Daily load (Watt hours) X Inverter Efficiency (%)= AC Average Daily Load, Adj (Wh) + DC Average Daily load (Wh) / DC system Voltage =           Average AH/day X Days without sun X Battery discharge limit (%+1) Battery AH = Number of batteries in parallel           DC system voltage / Battery Voltage = Batteries in series X Batteries in Parallel = Total batteries

With the load and battery bank considerations, the solar panel array is sized:

Array Sizing

Average Ah per day X	Battery Efficiency (%) =	Adjusted Average Ah per day /	Peak sun hours per day =
Peak Array Amps /	Peak amps per panel /	Panels in parallel
DC system voltage /	Panel voltage =	Panels in series
Panels in series X	panels in Parallel =	Total system panels

That is it, lots of load research, figuring and adding involved. Do it right the first time and it won't need to be done a second time, which is always more costly.