Heat Pumps vs. Solar Pool Heaters

A common thinking  in the PV (photovoltaic) dominated world of solar today is that you might as well use excess electricity from your PV system to power a heat pump to heat your pool eliminating the need to allocate space on the roof to an unglazed solar pool heater.  The PV company wants all the space on the roof for PV  but IF you are a pool user who will heat your pool you have to consider this question very carefully.  A heat pump sized to equal the output of the smallest gas heater will require 50 amps at 220VAC. That is a significant burden on your electrical panel.  An upgrade may be required rendering the idea moot before we even start looking at energy numbers.

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  A Hot Sun solar pool heater will outlast a heat pump. The warranty is 3 times longer. A solar pool heater has no moving parts other than  a $30 vacuum breaker and a pool industry standard motorized valve.  

Heat Pump Energy Produced

Heat pump effectiveness is a key consideration. COP (coefficient of performance) is the energy delivered divided by the electrical energy used. There is no standard of testing for comparison. It is up to individual heat pump manufacturers to specify their own numbers. The very optimistic number we see claimed is a COP of 6 but note this is at a pool temp of 80F, a humidity of 80% and an air temp of 80F. Humidity is a key consideration in two respects. A pool in a humid environment doesn’t lose much heat.  The largest mechanism for heat loss from a swimming pool is not convection or radiation, its cooling from the water evaporating. That is why a cover on a pool significantly reduces heat loss and most pool covers don't have much insulating value.  If the air above the pool is humid then the tendency for the pool water to evaporate is reduced.  Its similar to the more obvious idea that if two objects are closer in temperature, the heat transfer between them is less.   The heat required to maintain a pool temperature in a humid environment is much less than one in a dry environment  so a heat pump may provide adequate heating power. Secondly the COP of a heat pump drops off sharply when the humidity is lower. Humidity in season is high on the east coast and low on the west coast of North America. This is why we don’t see heat pumps being used in California but we do in Florida. A heat pump simply can’t keep up with the heat loss on a west coast pool unless the season of use is short and restricted only to the middle summer months and the pool uses a cover and the pool gets good direct sun and is protected  from direct ocean winds. Our collective history and the market itself over the last decades is evidence of this fact.  Heat pumps have not penetrated the market on the west coast. Their market exists on the humid east coast.  Its interesting to note that the electricity consumed is the same in either case, the difference being the output of heat is much higher in a humid environment.

 If you are in a humid environment and you have 50 amps available at your electrical panel then a heat pump powered by excess electricity from your PV array is a consideration.  If we consider PV operates at roughly 20% solar efficiency relative to the space required and solar pool heaters will operate anywhere from 50 to 85% its apparent that with a heat pump COP of 3 to 5  we are delivering the same energy to the pool with the same roof top area, PV vs solar pool heater.   To beat the performance of a solar pool heater with a heat pump the additional size of the PV array will be in the neighborhood of the size of a solar pool heater which for an average 16x32 pool might be something like 500 sq ft.  That’s about 40) 250W solar modules.  That’s your whole PV array and more  doing nothing more than powering the heat pump.  The smart approach is to allocate space to the solar pool heater. It is a far more elegant, cost effective and powerful pool heating option rivaled only by natural gas heating and nobody wants to soil the glory of a solar powered sustainable energy home with a gas flue chugging out carbon all day long.

Heat Pumps Compared to Gas Heaters

The heat pump steals about 3 to 5 parts energy from the outside air compared to the electricity required to drive it (the coefficient of performance or COP is 3 to 5)  depending on air temperature, water temperature and humidity. For reference , consider that if natural gas costs 3 times less than electricity then the heat pump costs the same to operate as a gas heater  when its COP is 3. Heat pumps are less powerful, present higher  upfront cost and have an advantageous operating cost only when conditions are favorable. Gas wins but it’s a fossil fuel burner and its less reliable and won’t last as long. To get off gas and maintain that sustainable home label, gas is out..  Again due to humidity considerations, heat pumps are a reasonable choice on the east coast but gas wins on the west coast and in either case solar pool heating has an appeal if and only if you can find the space to do an adequately sized system without risking roof damage or devaluing the home. Enter Hot Sun. 

Solar Makes Sense

When sized correctly for the situation a solar pool heater delivers a longer swimming season than a gas heater or a heat pump and  at no operating cost or noise (except upon start up when the air flushes through).  It is worth noting that the use of gas at a lower setpoint to augment solar or the other way around  renders the economic decision in favor of the natural gas heater because you are not going to be able to save enough natural gas dollars to justify the solar addition. If you add solar to a pool that is heated with natural gas you give yourself the option to heat freely at no cost. Natural gas can be the back up but remember that if you leave gas running constantly you will actually be taking away from what solar could do so its best to set gas as low as possible and lock it out at night and through bad stretches of weather.  More energy is consumed maintaining a pool temperature than letting the pool temperature drop through bad weather and then recovering when pool use demand occurs.  Solar does this naturally of course.   Since heat pumps and solar heaters operate under similar conditions and heat pumps are slow to heat, using a heat pump and solar together doesn't make any sense. At least gas can be used to boost pool temperature a few degrees in a few hours quickly  Note there are many markets where the cost of natural gas is much higher like Hawaii. Solar pool heating is very popular in Hawaii. Also natural gas is not available everywhere and against more costly  propane, solar wins out easily as well.   From a strictly economics perspective solar is not an energy saving technology. It is a pool heating choice. Unless you do it yourself, the cost of solar pool heating is too high to justify against natural gas costs alone but against the cost of a natural gas heater or heat pump, solar gets a lot more attractive financially.    Its not a question of which is the better choice. It is a question of how viable the site is for solar pool heating and can we find enough space to do what we want.   That is not always a given. Hot Sun’s unique custom fitted approach where we can take advantage of piecemeal spaces with irregular borders allows us to capture the maximum solar gain from the available space. Don’t use the north facing space unless the pitch is very low. Flat roof spaces are great because we are out of the direct wind. This improves our performance as much as the loss due to sun angle. Hot Sun will consult with you looking at google earth mages and street views to help evaluate your best options. 

 

 

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