What is the best solar pool heating method?

Keeping a swimming pool heated can be a big energy expense. Fortunately, there are both low-tech and high-tech methods with solar to save money and be environmentally friendly.

Photo of a swimming pool

Do you have a swimming pool? Nice! It’s not very pleasant to take a dip when your pool temperature is in the 70s, is it?

The upfront cost of equipment to heat a swimming pool with traditional methods - natural gas, propane, or electricity - can be relatively low, but you will quickly learn that the monthly fuel cost for keeping 10,000 gallons of water at a nice temperature adds up quickly.

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Just like you can save money and help the environment by using solar to power your home, you can also use solar to heat your pool. There are a number of ways to do this, from very low-tech pool covers to high-tech heat pumps powered by photovoltaics. Which one is right for you? Depends on your budget, heating needs, and how complex a solution you’re prepared to deal with.

This article will list several methods for solar pool heating, from cheap and low-tech to pricey but sophisticated.

What every pool owner should use: solar pool covers

Did you know that 50% or more of the heat lost from a swimming pool is through evaporation?

While pools do lose some heat through radiation, the majority of heat is lost because liquid water turning to water vapor uses a lot of heat energy. This is the same reason why sweating is so effective at cooling you down, especially in a dry climate.

Swimming pool covers keep your pool warmer by blocking evaporation. While a cover is usually made of multi-layered plastic with air pockets that help the cover to act as an insulator, blocking evaporation is the main reason why they work.

Saving water is another benefit, especially if you live in a region prone to droughts and water shortages.

There are many different kinds of pool covers, from roller mounted ones for in-ground pools to large covers that you can drag on top of an above-ground pool. That’s a topic outside the scope of this article, but I thought it’d be worth mentioning one particularly nifty idea: solar pool rings.

Solar pool rings are basically just plastic discs that you toss on top of your pool when you’re done for the day. You buy as many as you need to cover your pool. While they don’t cover your pool completely and therefore won’t keep your pool as warm as a full cover, they have the significant advantage of being cheap and easier to handle than a single big cover.

Each ring might weigh only a couple pounds, making them easy for a smaller person to place and remove.

Here’s some examples from Amazon:

Lower tech solution: polypropylene solar pool heaters

One step up in pool heating technology is a solar thermal collector. These devices don’t have any moving parts, but are simply a continuous tube of black plastic (usually polypropylene) arranged into a flat collector.

DIY potential
Lowest cost
Good heating potential
Surprisingly long lifetime
Can be installed and put away for when you need it
May require a pump upgrade
Out-of-the-way installation becomes more complex

With tubing, you connect the solar heater to your existing pool pump and point it at the sun. The water from your pool circulates through the tubing, collecting heat energy along the way.

If you’ve ever left a garden hose sitting in the sun, you know how quickly the water inside can heat up. These solar heaters work on the same principle. Despite the simplicity (they’re basically a continuous plastic tube), they can be surprisingly effective.

To increase the heating potential of the system, simply connect multiple heaters in series. If you want the option to stop heating, you’ll need to install a control valve into the flow.

Here’s some examples from Amazon:

The main advantages of this type of heater are simplicity and effectiveness. Solar thermal collectors have a potential efficiency of around 50%, compared to the best photovoltaic panels that max out around 23% efficiency. This means that about half the sunlight that hits one of these collectors can be captured as heat.

Because they’re simple, you could install them only when you need heating. For example, if you have a pool party planned, install the panels the day before or in the morning and simply lay them in the yard. When your party is over, you can uninstall the heater and put it away.

A permanent or semi-permanent installation that isn’t ugly is a little more challenging, but can still be a DIY project. If you have an above-ground pool, you could simply mount your collectors on sheets of plywood and prop them against the outside of the pool.

Alternatively, you can place them on a nearby rooftop such as your house, garage, or a pergola. That’s more complex, and adding more collectors and tubing increases the back pressure on your pool pump, potentially to the point where you need to upgrade your pump.

Slightly more high tech solution: plastic solar thermal panels

One step up in heating efficiency are products that are similar to the polypropylene solar thermal heaters above, except that they are rigid panels with a clear plastic front.

DIY potential
Low cost
Good heating potential
Can be installed and put away for when you need it
May require a pump upgrade
Often aren’t designed for a rooftop installation
Fewer product choices
Less durable and short warranty (often 1 or 2 years)

The clear plastic enclosure helps these devices trap more heat, effectively turning them into little greenhouses. Otherwise, they operate and are installed in the same manner as the simpler solar heaters listed above: they fit into your existing pump system, and you can link multiple heaters in series to increase the heating capacity.

Here are some that you can find on Amazon:

If these heat better than solar thermal heaters without an enclosure, why wouldn’t you always choose this type? The main reason is that the cheap plastic frames of these devices aren’t very durable. The devices listed above on Amazon only come with a 1 or 2 year warranty. In contrast, the polypropylene heaters listed have 10 or 12 year warranties.

Because of this, you may find after a couple years of use that they start to leak, or the legs or kickstand breaks, or the housing has cracked. In fact, if you read the Amazon reviews for these devices, these are common complaints. In contrast, the polypropylene heaters often have 5 star reviews.

Still, they’re a cheap solution. Because of this, they can still be a good choice if you need low-cost effective solar pool heating.

High tech solution: home solar photovoltaic system with pool heat pump

If you have a big swimming pool, want to heat your pool even if it isn’t sunny, or you want more precise temperature control and powerful heating, the big high tech approach is to install a home solar PV system and a heat pump for the pool.

Steady pool temperature
High heating capacity
Long warranties: 25 years for solar panels, 10 years for heat pump is common
Eligible for solar incentives
Good financial payment for many homes
Can be retrofitted to an existing home solar system
Not a DIY project
High upfront cost

If you’re brand new to the idea of solar photovoltaic panels, you can read our Solar 101 article or the complete guide to buying solar for your home.

In a nutshell, solar PV panels convert sunlight directly into electricity. Your home will remain connected to the electric grid, which means that when you need more electricity than your panels are generating, you can pull power from the grid.

On a sunny day when you’re not using much electricity, any excess power will be sent into the grid (and you’ll receive a credit on your utility bill).

To heat your swimming pool with solar PV panels, you will get a heat pump installed. If you’re not familiar with these, you can think of a heat pump as a backwards air conditioner: it uses refrigerant and a compressor to move heat from one place to another. But unlike A/C, a heat pump moves heat from outside into your home - or, in this case, into your swimming pool.

(By the way, a home heat pump can work both forwards or backwards - meaning, it can work either in a cooling or heating mode, which is handy if you’re trying to go all-electric in your home.)

Because a heat pump runs on electricity and your solar home will remain connected to the grid, it doesn’t matter if it’s sunny when you need to heat your pool. The power will come either from your solar panels or the grid. There will be times when your panels will produce more than you need, and the rest will go into the grid. On balance over the course of the year, this give-and-take from the grid can often mean that you can get a net of 100% of your electricity from the sun.

A heat pump can be sized so that it has the capacity to always keep your pool at the desired temperature, and will also be thermostatically controlled so that the temperature automatically stays there.

Sounds great, right? It is, and this is the ultimate if you want to keep your pool comfortably heated without using fossil fuels.

As a big plus, there are many incentives available for both solar photovoltaics and heat pumps. Use the Solar Nerd calculator to find solar rebates and tax credits in your city, and check with your utility company to find rebates on heat pumps.

The downside: this is the most expensive and complicated solution. However, if you’re thinking of going solar anyway, this can also be practical as you can simply add additional panels to your system for the additional pool heating load.

The process of getting one of these systems installed is the same as getting any home PV system installed. Use The Solar Nerd to get quotes from local solar installers and read our guide to solar to learn all the basics of what you need to know.

Ask the installer if they can install the heat pump for you. Many installers, especially in states where pools are common, will either have in-house expertise to do this for you, or partner with a local HVAC company. This makes it more likely that your PV and heat pump will be sized correctly and work well together.

Of all the ways you can heat your swimming pool with solar, one technology that we can’t recommend is rooftop solar thermal using flat plate or evacuated tube collectors.

These aren’t the cheap and simple plastic heaters described above, but sophisticated solar collectors that are used to provide hot water or heating for homes.

Instead of cheap black plastic, these rooftop solar collectors use metal frames, anti-reflective glass, heat exchangers, and sometimes vacuum tubes to achieve very high efficiency. The best solar thermal panels can convert 70% or more of the incoming sunlight into useful heat.

So if these panels are so efficient, why aren’t they recommended? There’s a few reasons.

The main reason is cost. Solar photovoltaics based on silicon have seen the same kind of price drops that have made computer chips better and cheaper every year. In 1977, the price of solar PV cells was $77 per watt. Today? Only $0.13 per watt!

Solar thermal, however, is a technology based on glass, steel, and pumps. There isn’t the same opportunity for technology advancement. While solar PV gets a little cheaper every year, solar thermal stays about the same price.

Because solar thermal isn’t usually a DIY project, you’ll need to pay a contractor to install the system. There are relatively few companies that install solar thermal, but thousands around the country who are dedicated to rooftop solar PV. This means that you will probably struggle to find a good local company with experience in solar thermal, let alone multiple companies so that you can get competing bids.

The other reason is durability. Solar PV is a technology with no moving parts. Because of this, you can get a system with a 25 year product warranty on the panels and a 25 year warranty on microinverters. Heat pumps too, because they are based on the same principles as air conditioners, are a mature and robust technology.

The technology of solar thermal, on the other hand, is mechnical and involves very hot fluids, heat exchangers, pipes, and pumps. If you’ve been a homeowner for more than 10 or 15 years, there’s a pretty good chance that you’ve needed to replace your hot water tank. You can probably expect a solar thermal system to have similar reliability before components need to be repaired or replaced.

Bottom line: many options at different price points.

The great thing is that you can have a lovely swim in your backyard pool at a comfortable temperature using the power of the sun, and you can do it with several product choices that range from very affordable to quite expensive and sophisticated.

If you decide that you want to kill two birds with one stone and supply both your home electricity and pool heating needs with solar panels, use The Solar Nerd to get competing bids from high quality, local contractors who can help you out.

#Heat Pumps #Solar Thermal

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