Heat large pool with electric via solar panels?

Question

Where might I go for help with this? Should I try to price it out myself or find a contractor? What type of contractor should I look for?

My community of 60 families owns a pool. It is about 30' x 60'. It is heated by gas now, and the gas is the major expense of the pool each year. We spent $7,500 for gas last year.

How might we go about evaluating the costs and other pros and cons of switching from gas to electric heat? The current gas heater is about 1/2 way through its expected life. The goal is to reduce the annual cost for energy.

We are located in NY and there is a 1/2 acre grassy area near the pool that is communally owned and which is not used much because it is gets boggy. Perhaps we could put some solar panels there.

I guess we could estimate the amount of electricity needed to heat the pool from the $7,500 for gas.

How do we calculate the number of solar panels needed?

Can solar panels be put close to the ground? I have only seen them on roof tops.

I estimate we use the pool 4 months a year.

Update: I've heard that you can "bank" electricity you make over what you use. Since we will use the electricity in only 4 months, the electricity we make in the other 8 months can be banked and used for free when we need it.

Answer 1

PV panels are ~15% efficient. Hot water panels are much more, and probably much cheaper.

Way more efficient to just heat the water, rather than convert to electric and then to heat. Maybe with the exception of a heat pump... $$$

Answer 2

This has already been said in one answer - but I want to say it again so that it is very clear. Generating electricity from solar only to then turn around and run electric heaters for the pool is not an efficient way to go at all. Overall such system will likely be less than 10% efficient in terms of solar energy conversion. Go with a solar water heater system instead. A well constructed system should be able to convert 3 to 4 times as much of the available solar energy.

Note that many people have built their own solar water heater systems that you can find online.

Answer 3

How much energy do you need?

In four months you spend $7500, so assuming you heat using natural gas, that would indicate a consumption of about 19,000 cubic meters. At 10.8 kWh per cubic meter, we're talking 200,000 kWh; in four months that's an average power requirement of 70 kW.

Can the Sun help?

Reasonable solar output in New York is around 5 kWh/m² per day.

Four months is 120 days, so solar panels will give you 120*5 = 600 kWh per square meter. To reach 200,000 kWh you would then need 340 square meters, i.e. double the surface area of the pool.

The cost of such an installation can be quite high.

Do you really need all that energy?

Now 70 kW divided by the surface is around 430 W, which seems to indicate appreciable evaporative losses throughout the day. Evaporative loss is a function of three parameters (water temperature, available surface and airflow temperature and speed). You may get considerable savings by:

• covering the pool with a black "solar blanket" when not in use. If you do not already do so, this should cost around $1,000 and can save you from about one third to one half your current energy budget (even more if you don't use the pool every day and leave it uncovered). That's around $2,500 each year, and a good solar blanket will last 5 years easily.

• surrounding the pool with a tall fence, hedge or windbreak mesh that will lower wind speed and reduce losses when the pool is in use.

Solar heating

Using electricity, which is one of the "noblest" forms of energy (as it can be easily transformed in any other form) to generate heat (which is the basest form) should be a last resort. Photovoltaics lose around 85% of the available energy before even starting.

Much better to directly tap into solar heat.

To actively heat the pool, depending on the distance between the pool and the green area, my suggestion would be to use either a water recirculation system (possibly connected with the pool pump) or an insulated circuit with heat exchanger (requires additional, low power pump) between the pool and a solar heater plate. This is basically a long black UV-resistant PVC tube where pool water or heat exchanger fluid (water plus glycol and other substances) slowly circulates, and gets heated by the sun. Thermal efficiency is near 80-90% compared to the 10-15% of solar panels, i.e. you can get by with a surface four times smaller: half the size of the pool instead of twice that. Also, the cost is way lower, and the components are next to worthless (PVC tubing, come on), which means they're really unlikely to attract unwanted attention even in not-so-safe neighbourhoods. Insurance costs are proportionally lower, and repairs can be done by anyone.

Here you can find more info, including DIY instructions.

(Note: you need to drain the tube completely during the cold months, lest it be damaged by freezing).

Electricity banking, heat pump

Both methods are "multipliers" for the efficiency of a photovoltaics system. With electricity banking, you sell (or "store") electricity into the utility grid in those months when you don't use the pool at all. Then when you have need of the pool, you divert the PV output to the pool, and can supplement with energy from the grid - that you "stored" in the past months.

Unless you get special tariffs, the best you can hope is to get the energy at the same price at which you sold it; this is pure banking. But the electricity was produced in the coldest and darkest months, so the split won't be exactly 8:4; more likely something like 50%-50%. So the energy you can get back is of the same order of magnitude of that which you produce in the summer. This then doubles your efficiency, and you can use a PV surface half the size: i.e. the same area of the pool.

Same goes for a heat pump. A heat pump uses, say, one unit of electricity to "move" some units of heat from the environment into whatever you need to heat; the electricity also adds to this heat. Typically you get another doubling of efficiency, even if the heat pump requires maintenance; so your heating costs won't be really halved.

Combining electricity banking with heat pumps, you could then get to heat a 160 m² pool with 80 m² of solar panels. It seems to me that it's still better to go with solar heating tubes, even without factoring in maintenance costs, permits and installation.

Answer 4

You have a pretty big pool by residential standards but I still am surprised it is costing you that much... almost $2k per month!

Assuming a price of $.90 / therm (average in the NY area, according to some random site I found) and the numbers on this US Energy Star page, that's more than double what you should be spending for a 1800sf pool in New York.

What temperature are you setting the pool to? Do you run the heater full-blast, 24/7? Do you have an unusually low-efficiency heater?

I'm guessing you don't have a pool cover, if you don't that would be an excellent place to start.

Answer 5

To reduce your energy costs you have a few options (some taken from comments and not from the post).

First if your temp is set to 83.5 that is the largest part of your problem. A lower temperature would be desirable. The "best" range for swimming is 77 - 82. If your not swimming turn it down to the 60s or lower. If you are swimming then 77 in the winter months sounds good.

Next look into a good pool cover. The problem with pool heating is that your heating a big area of water which 90% of the time your not even using. Using a good pool cover will help reduce those costs a lot. An automated cover may even be an option.

Take a look an enclosing the pool. A plastic "green house" style enclosure may save quite a bit. It may also extend the time your able to use the pool (into the winter). If it's in an enclosure now, check your options for switching the panels in the enclosure. Nothing heats water better then the sun.

To answer your question directly, you need to hire a pro. Someone that will come and evaluate your situation, and lay out your options. There are many, and replacing your current heating system with a new one may be an expense that you don't recover for many many years.

Answer 6

As previously stated, heating the water directly is a lot more efficient and cheaper.

I'd like to add that it's also a lot less complex of a system. You only need (a lot of) tubing, paint it black and that's it. Replacement parts are easy to come by, both in terms of availability and price. Possible repairs in the future do not require expert knowledge and can be executed with only little DIY skills. This could be done by community members which reduces costs and adds a possible social aspect as the community is able to maintain their pool.

Answer 7

It might not be the most aesthetic solution, but a better way to heat your pool with solar power is to paint it black rather than white/blue or whatever it is.

Or spread black foil over /under the surface when not in use.

I've used that for a small (3m dia, 0.6m deep) pool, very effective and low maintenance (as long as you ensure water flow at both sides). For a large pool, I'd arrange for something that lets the foil be rolled and left in a corner. Or you could just leave it at the bottom.

[edit] to comments: yes, this admittedly isn't "the best", but still better than photovoltaics. Perhaps you already have - or need - a cover for the pool, might as well make it black.

[edit2] To be clear: I mean that this is superior to having an expensive photovoltaic array convert light to electricity, then that to heat. This goes directly from light to heat.