Solar Hot Water

Solar hot water heaters use the heat from the sun to heat water. A solar hot water system is composed of a solar collector, a water tank, and connecting pipes. With the exception of batch heaters/ICS units, solar hot water systems work by cycling water from a storage tank to the collector, which heats the water.

Why Heat Your Water with the Sun?

Solar hot water is a good idea for a variety of reasons, including:

• Heat from the sun is incredibly abundant, and will be available for millions of years unlike heat from natural gas, which depletes a non-renewable resource, and contributes to air pollution and climate change.
• Heat from the sun is free, so there is no monthly cost for using solar hot water, only the initial cost and maintenance of the collector.
• Solar hot water integrates perfectly with conventional hot water systems (especially instant water heaters), so you won't sacrifice any reliability.

What are the Types of Solar Water Heaters?

If you are looking to install solar hot water, you have three main choices in solar hot water collection systems: flat plate collector, integral collector storage, and evacuated tubes. Which system is right for you depends on where you live, when you use hot water, and the strength of your roof, so choosing the right system can get complicated. Before buying any system, you should try to get a professional opinion. However, here are some steps you can follow to get a general idea of where to start.

Three Main Types of Collectors

• Flat Plate Collector: probably the most common type of solar collector, it is characterized by a flat, rectangular shape, which collects the sun's heat and transfers it into the water pipes that snake through it. Either "passive" or "active" circulation.
• Integral Collector Storage (ICS): a large insulated box with a glazed glass top encloses a dark colored water container. The suns shines through the glass and heats the water in the dark colored container. The insulation and glazing keep the heat in for most of the day. A "batch" collector.
• Evacuated Tube Collector: evacuated (i.e. vacuum) tubes insulate metal collector plates, which allow the sun's heat to boil a low-boiling point liquid. The resulting hot gas floats up the tube, and transfers its heat into a pipe of flowing water through a heat exchange bulb. "Passive" working fluid circulation. Usually "active" water circulation.

How Do I know What to Buy?

To buy a solar hot water system, you need to know two things:

1. How much water you use.

2. How much water the solar hot water system will provide.

1. How much water do I use?

Figuring out how much water you use is fairly straightforward. First, find out the following information:

• The flow rate of your showerheads (number of gallons used per minute)
• The temperature at which you shower.
• The average length of your shower.
To get these values for a large building with many showers, click here
• If you are sizing a solar hot water heater for a large building, you can estimate the length and temperature of each shower using HOBO Data Loggers, which measure temperature. Attach a sensor to each showerhead so that when the water is turned on, the water stream will run over the sensor. Record the data for 1 -2 weeks. Once you have this data, you can determine the length of the shower, when most showers occur, and the temperature at which people shower. A spreadsheet is coming soon to help you analyze the data from your HOBO Data Logger.
• The amount of water your dishwasher uses per cycle, and the temperature at which your dishwasher runs.
• The amount of water your washing machine uses per cycle, and the temperature at which your washing machine runs.

How can I find this information?

Once you have these numbers, use them to figure out how much water you use in an average day. Keep in mind that some of the water used by your showers, dishwasher, and washing machine is cold water, so this number is likely to be an overestimate of the amount of hot water you use and you should take that into account when performing these calculations. A spreadsheet to help you perform these calculations is coming soon.

Now you know how much hot water your solar hot water heating system will need to supply!

2. How much water will my solar hot water heating system provide?

To figure the energy supplied by a solar hot water heating system, follow these steps:

• Download the TMY data for your area from our website. (coming soon)
What is TMY data?
• TMY (Typical Meteorological Year) data provides hour-by-hour information on temperature and insolation for a location during a given year.
• Put the TMY data into our excel template to get average ambient daily temperature and insolation.
• For flat plate collectors and evacuated tubes, find the following data (usually found in the technical specs section) and enter it into our spreadsheet:
• FR ta value, also called Y-intercept
• FRUL value (slope)
• Kta (incident angle modifier)
• The area of the solar hot water collector (for example, if a flat plate collector is 8 ft x 4 ft, the area is 32 ft (2))
• Use the results from our spreadsheets to choose your solar hot water collector based on which is most efficient at the times that your hot water demand is largest.
• For ICS systems, the analysis is different. Soon, we will have a way to help you analyze ICS systems to determine if they are right for you.

Now you?e done! You have all the information you need to size a solar hot water heating system for your building. Just choose the collector that is most efficient at the time of day you need your hot water, and size it to supply as much of your hot water demand as you can while still being cost-effective. There are still some details to work out for example, do you want a direct or an indirect system? but these details are better for the supplier to help you with because the supplier has experience working in your climate.

How Much Will It Cost? How Much Will It Save Me?

On average, solar hot water reduces the cost of your water heating bill by 50% - 80% (3). Use the spreadsheet that you will soon be able to download from this page to do more specific analysis on the solar hot water heater that you are considering

Other Terms You Might Hear Related To Solar Hot Water:

Methods of Circulation:

• Active System: water is actively pumped through the system by a pump.
• Passive System: water is moved through the system by natural convection.
• Batch Collector: water sits in the collector, and is only circulated when you draw water from it.

Methods of Heating:

• Direct: a system that heats water directly. These systems are good in warm climates.
• Indirect: a system that heats an antifreeze, which in turn heats the water once it is circulated through the water tank. Indirect systems are good in cold climates where freezing water has the potential to break the pipes.

Author: Lauren Leonard

Contributors: Nick E

References: (1) Energy Star, (2) Energy Star, About Housekeeping,(3) US Department of Energy, Energy Efficiency and Renewable Energy

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Lotus Photography by cjn