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SolarTown Learning

Consumer Guide for Solar Pool Heating

10-23-2009
Category: Solar Pool

Choosing Solar for Your Pool Water Heating Needs

Heating your pool or hot tub with a solar pool heater is a proven simple, affordable, and reliable way to save money and help the environment. Whether you use your existing pump or a solar pool pump, you can connect a solar thermal water heating system to any pool or hot tub to provide low-cost heating that will extend your outdoor swimming season and save you money. But knowing how large your solar thermal system should be and how to keep it functioning at peak efficiency requires further explanation. SolarTown is here to help with this practical guide for solar thermal pool systems. Solar pool heater

Determining Solar Thermal Water Heater Size

Sizing your solar hot water system to suit your swimming needs will depend on a variety of factors:

  • The size of your pool
  • The number of weeks or months you use your pool in an average year
  • The average temperatures in your area
  • Your region's available solar resources (refer to the US Department of Energy (DOE) for a good estimate)
  • The efficiency of your solar collector system
  • And whether or not you employ a pool cover

For a quick estimate of a cost-effective size for your solar water heater system, you can use this rule of thumb: take the surface area of your pool and multiply that by the percentage of the year you'll be using your pool. Obviously, in cooler or cloudier areas, the ratio may need to be adjusted if you want to provide a larger percentage of your total pool heating requirements.

For instance, if you live in Florida and intend to use your pool year-round, then your solar collectors should be roughly the same surface size (100%) as that of your pool's surface area. This size should be sufficient to provide 100% of the water heating you require for the entire year.

If, however, you live in California and only wish to use your pool 6-8 months (50% to 66%) of the year, then your solar collectors should be about 50% to 66% of the surface size of your pool. A solar water heating system this size will likely need to be supplemented by another heating source, but it is a cost-effective size for the number of months your pool will be used.

Solar Pool Heaters for Hot Tubs and Spas

Heating a hot tub or spa is a little different because of the higher temperatures and the relatively small size. That said, you can generally count on needing about a 4' x 8' solar panel for a 300-gallon hot tub and a 4' x 10' panel for a 500-gallon hot tub (depending on your collector's efficiency, of course).

In all cases, regardless of where you live or whether you're heating a pool or a hot tub, you can lengthen your swimming season and decrease the number of solar panels required by using a good quality pool cover. We have had some pushback from some of our customers about installing a pool cover, and yes, we understand some of the inconvenience of these pool covers, but pool covers are the most efficient use of your money to significantly reduce both water loss and heat loss due to evaporation.

Solar Pool Heaters for Above Ground and Inground Pools

Water heating requirements will differ slightly for above ground and inground pools and there are different systems available for a solar above ground pool and a solar inground pool. This is due largely to the susceptibility of above ground pools to lose heat more readily than in-ground pools. With a lower level of insulation in above ground pools (inground pools have the benefit of being insulated by the surrounding soil), they tend to need more frequent replenishment of hot water. The insulation level of your above ground pool will depend somewhat on the materials it is made from-aluminum, steel, concrete, gunite, resin, or fibreglass. Each of these has a different ability to maintain the heat in your pool. Keep this factor in mind when calculating the size of your solar thermal system.

Savings Potential with a Solar Thermal Water System for your Pool

If you currently have a non-solar pool heater, you can estimate your potential energy cost-savings achieved with a solar pool heating system by using the following equation (using figures provided by the National Solar Test Facility (NSTF) in Mississauga, Ontario):

Annual Dollars Saved = [Rated Annual Energy (GJ/m2) x Individual Collector Size (m2) x Number of Collectors x Cost of Energy ($/GJ)] / Efficiency of Existing Pool Heater

When using this formula, we make several assumptions:

  • The average efficiency for propane, oil, and natural gas heaters is about 0.75 and 1.0 for electric-resistance heaters.
  • Your manufacturer should be able to provide you with an average efficiency for your solar water heater, but to calculate this yourself, refer to the US DOE's solar thermal performance formula.

Obviously, the cost of your energy will depend largely on the type of fuel you currently use to heat your pool. To estimate the $/GJ (gigajoules) cost of heating your pool, use the following figures:

Fuel

Unit Cost of your Bill

To get $/GJ, multiply by:

Natural Gas

_____cents/m3                

0.2686

Oil

_____cents/litre                

0.2585

Propane

_____cents/litre                

0.3917

Electricity

_____cents/kWh

2.778

Here's an example of how this equation might work:

Rated annual energy of your solar system:          1.3 GJ/m2

Individual collector size:                                              3.6 m2

Number of collectors:                                                   6

Cost of energy (natural gas):                                      19.9 cents/m3 x 0.2686 = 5.345 $/GJ

Efficiency of existing pool heater:                           0.75

With these numbers, your equation will look as follows:               

[1.3 x 3.6 x 6 x 5.345]/0.75 = $200.12 annual dollars saved

Getting the Most out of your Solar Hot Water Heater

Siting your solar collector for use with your solar swimming pool is critical if you want to harvest the most solar energy possible. Typically, there are two main site factors that will affect your solar heater's efficiency: its orientation and tilt in relation to the sun.

Regardless of whether you install your solar thermal water heater on your home's roof or near your swimming pool, you should be sure to place it where it will receive maximum exposure to the sun. Orienting your solar collectors to the true south (if you live in the northern hemisphere) is ideal for maximum daily and seasonal solar energy production. Also ensure that your solar collectors are not obstructed by things that will shade it, such as trees, power lines, other roof lines, buildings, and other landscape features.

Although the angle of your solar collectors will not significantly reduce your solar collector's efficiency in heating your pool water, it can impact how much hot water you're able to produce, so paying attention to this detail will boost your system's performance. The tilt of your panels is determined largely by your latitude and how many months you'll be heating your pool water. In general, year-round solar water heaters should be tilted to equal the angle of your latitude. However, if you intend to heat your pool water only during the summer, than an ideal tilt is your latitude minus 10⁰-15⁰.

Rebates and Incentives with Solar Thermal Pool Systems

Today, there are numerous programs for making the purchase of a renewable energy system like a solar thermal heater more affordable. With all of the rebates and incentives available to consumers and building owners, there's never been a better time to invest in clean, solar energy.

First, let's differentiate between the various types of financial programs provided by various levels of governments (which is also summarized by the Database of State Incentives for Renewables & Efficiency (DSIRE)):

  • Personal tax incentives are those that include income tax credits and deductions offered by many states.
  • Property tax incentives include exclusions, credits, and exemptions that provide that the added value of a renewable energy system is not included in the valuation of the property for taxation purposes.
  • Rebate programs offered by many states and municipalities, as well as utility and electric companies, provide funding for the installation of a new renewable energy system.
  • Sales tax incentives are those that exempt the purchase of a renewable energy system from state sales tax.

Unfortunately, the federal tax incentive for solar water heaters used for pools and hot tubs does not qualify for the 30% credit offered for other solar thermal systems, however, many states, municipalities, and local utilities provide rebates and incentives for purchasing a solar thermal system for your pool or spa. You can search by state using DSIRE's Renewable Energy Maps. Even if there's nothing listed there, you may want to check your own local utility's website to see if they offer their own rebate or incentive program. For other financing options for your solar water heater, you may want to consult with the US Department of Energy's Borrower's Guide to Financing Solar Energy Systems.

Solar pool systems allow you to substantially reduce your carbon footprint and not break the bank-while you enjoy your pool or hot tub at the same time.


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