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Components

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Solar water heating systems collect solar energy from the sun to preheat water so that your gas or electric heater — whether a conventional tank or tankless — does less work. A solar water heating system can be added to almost any water heater. But if your water heater is worn out, you can install a combined-tank system that incorporates the solar preheated-water storage and your water heater.

Unlike most solar electric systems, all solar water heating systems are ‘off grid’. The energy from the sun is stored and used locally. This is why it is important to size a solar water heating system to match your hot water use.

In an overcast sky climate like western Oregon, solar water heating is predominately sized to match the domestic hot water demand in the summer. In clear sky climates like eastern Oregon solar water heating can make a significant contribution to space heating systems that use hot water. Depending on those variables and whether your backup is gas or electric, the system would typically save between 1,500 and 3,000 kilowatt-hours, or 80 to 150 therms of energy, each year.

solar hot water diagram

A typical solar water heating system incorporates a solar collector, circulation pump, storage tank, controller, piping and various valves.  There are many different configurations possible, and the choice of design will depend on the purpose of the system, climate, space limitations and many other factors. Solar water heating systems have two main components: the collectors and the plumbing.

Collectors

In the Northwest, solar water heaters typically fall into two basic categories: flat plate collector systems and evacuated tube systems.

Flat Plate Collector Systems have a large, flat surface area (absorber) to maximize exposure to the sun, and has small tubes bonded to it. Fluid runs through the tubes, collecting the heat from the absorber. The sides and bottom of the collector are well-insulated, and glass on top completes the insulation. Water is pumped up through the collector when there is enough sunlight to heat the coils. When conditions are not right for heating water, the pumps turn off and the water drains back to a reservoir to prevent freezing and overheating. Several factors make the collector as efficient as possible. One is the coating on the absorber, which is specially formulated to both absorb as much heat as possible, and to radiate back out as little heat as possible. Another is the glass, which is high-iron and specially coated to let as much light energy as possible through and also prevent as much heat loss as possible.

flat plate collector system
Example of what a flat plate collector looks like

Evacuated Tubes Systems has a long, skinny absorber that is inside a glass tube. The tube has the air evacuated out of it, which makes it highly insulated—not too different from a thermos used to keep drinks hot. These systems also rely on pumps to move the fluid. The advantage is reduced heat loss from the collector to the surrounding air. This would be important in a climate with cold daytime temperatures where solar heated water is being used for space heating. Evacuated tube systems are generally more expensive than flat plate collectors.

evacuated tube system
Example of what an evacuated tube system looks like
CONSIDERATIONS:
  • Both the flat plate collectors and evacuated tube systems will typically be mounted on your roof.
  • Collectors should be located with a clear southern exposure for the best performance.
  • Solar water heater systems are more tolerant of shading and shadows than solar electric systems.

Plumbing

standard tank room
Example of a standard tank room

The plumbing of a solar water heater system is divided into two parts. The glycol system which circulates through the roof top collectors, and the potable water system which includes the solar hot water storage tank, the home water heater, and the safety valves and expansion tank.

The glycol loop consists of two insulated pipes running to the roof to allow a pump to circulate the fluid and bring the heat to the water storage tank. A heat exchanger separates the glycol loop from the potable water in the storage tank.

The plumbing in the “tank room” will contain a number of devices, a pump or pumps, temperature gauges, a pressure gauge, a flow meter, a controller, and ports to insert and check the glycol. On the potable side, the anti-scald valve and another temperature gauge will be plumbed in.

tankless solar water system
In this photo, the large tank is connected to the collector on the roof. The wall mounted tankless water heater will only turn on to heat up the water if it isn’t already hot enough.

Finding a location for a hot water storage tank can be challenging. Ideally, a location next to your hot water heater is best, however, a close location, even if through a wall in another room will work. Some homeowners replace their old tank type hot water heater with a wall mounted tankless water heater, and then use the old water heater space for the solar water heater tank.

Components

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A basic home solar electric system consists of PV cells connected and packaged together in weather-protected panels. Panels are fastened side-by-side on a racking system on a building’s roof to form an array.  When sunlight strikes the PV cells semiconductor material and bumps electrons off in a continuous stream, the array is producing direct current (DC) solar energy. That energy flows from the array through a metallic conduit down to an inverter. The inverter changes DC voltage to the alternating current (AC) for the household electric circuit powering wall outlets and all AC appliances.

While the solar array needs to be in direct sunlight, typically mounted on your roof, the inverter is generally mounted on the shaded north side of your home so that it doesn’t get direct sunlight and overheat.  For security reasons, interior locations, such as a basement or a garage may be chosen, but small, confined spaces such as a closet will not work due to the danger of overheating.  In addition to mounting the inverter, a DC disconnect switch and a utility meter will be located close by. The output of the inverter and PV system will be connected to a circuit breaker in your breaker box.

If your home is using electric energy (e.g. refrigerator, lights), solar-produced energy is consumed. Most residential solar systems are connected to the energy grid. If your solar system is making more energy than your house is using, the surplus gets sent back to the grid, banking the energy you produce for later use.

CONSIDERATIONS:
  • The solar array, which is made up on individual solar cells and panels, needs to be placed in an area which will receive direct sunlight.
  • The inverter needs to be close to the panels and connected to your house’s electric panel.
  • For the majority grid-tied solar systems, when the grid is down your system will be automatically shut off until the grid comes back online.  This occurs primarily for safety reasons.

Panels

Semiconductor material used in solar electric systems falls into two broad categories:  crystalline (hard/stiff) and amorphous (soft/flexible). Crystalline technology can be monocrystalline grown in a large crucible or polycrystalline grown and cast into a large ingot.  Both of these types are sliced into wafers and printed with electrical circuitry to become photovoltaic (PV) cells.  The cells are then electrically connected in a long series and assembled into panels. Panel sizes vary; generally speaking, a 260 watt panel would be about 15 – 20 square feet and weighs 35 – 40 lbs.  Conversion efficiency, the ratio of light energy converted to electrical energy, is in the range of 15 – 18%. Most of the solar installed in Oregon is crystalline.

Amorphous photovoltaic material generally comes in sheets.  Some amorphous material is transparent and can be applied to windows and other vertical surfaces.  A more common application is a narrow sheet that fits between the seams of a metal roof.  Amorphous PV is less efficient than crystalline panels, in that for the same area amorphous panels will convert less energy from the same amount of sunlight.  However, research is being done to improve conversion efficiency.

CONSIDERATIONS:
  • Amorphous systems require about twice as much space as crystalline panels, however it may integrate better with your roof.
  • Amorphous PV material shows up in many portable applications when being lightweight and flexibility is advantageous.

Standard Inverters

Inverters take direct current (DC) energy from solar panels and change it to alternating current (AC) that is used in buildings.  For systems that are tied to the electric grid, they must also match the frequency (hertz) within a very tight tolerance.  All grid tied inverters are tested to Underwriter Laboratory’s test standard UL 1741.

The inverter also contains safety and utility-interface circuitry, which enables a solar photovoltaic system to integrate and interconnect seamlessly and safely with the utility grid system and all the building’s electrical loads. Most modern inverters also contain data monitoring and web communication circuits so homeowner and system owners can keep an eye on performance.

Inverters generally work best when the panels of the solar array are oriented the same way with the same tilt. However, multiple inverters can be connected to your electric panel.  For instance, if you have a south facing solar array and west facing array, separate inverters can be used on the same house.

The advent and development of inverter technology is what made grid tied PV systems possible. Inverter technology continues to improve and warranties are getting longer.  Just a few years ago a typical warranty was 5 years, but now you can expect an inverter warranty of 10-20 years.

CONSIDERATIONS:
  • Inverters must be sized according to the output of the PV array.
  • Inverters should be located near the PV array, but in the shade.
  • Inverters have a low hum when they are working so they should be located in an area where this daytime noise is not an issue.
  • Unless you have batteries, the majority grid-tied solar systems will be automatically shut off with the grid is down. However, some inverters, such as SMA’s, will give you access to up to 1500 watts of power during a grid failure.

Micro Inverters

The advent of the new generation micro-inverter has opened an exciting era in small PV systems. It is now possible to install single panel grid-tie PV systems in apartments, condos, and small homes. For the cost of a little as a single PV panel ($500 for 200 watts) and a single micro inverter ($200), a PV system can be added to a home.

Micro-inverters mounted on each PV panel provide solutions for sites with shading issues or complex roofs with arrays mounted in more than one orientation. For example, in difficult installations where a shadow passes across the array during the day, a string inverter reduces production from all panels on the string. Micro-inverters minimize the effect of the shadow on power production by isolating the effects of the shadow to the individual panel.