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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 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. Even if you do have the capacity, the cost just to run a 50amp cable from the electrical panel to the heat pump can rival the cost of the heat pump itself.

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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.  

A solar pool heater requires roof space and a heat pump doesn't but if you are going to pay for the electricity to run a heat pump in California it is going to be very expensive. Where the heat pump idea gets some traction is from the fact that in some cases excess solar electricity from the PV system is available. If the electricity is free and there is no space for SPH and you can use the extra PV in the off season or sell it back to the utility then we have what seems like a compelling case for the heat pump. The truth is hidden in the details.

A typical pool heat pump operating at a COP of 4 (which may be generous) will consume 7.3kw and deliver 80,000BTU/HR. If we use as a sample case a 400 sq ft uncovered pool, the solar heater would be roughly 400 sq ft and at peak output would deliver about 120,000BTU/HR.  The heat pump can operate over a longer time per day but not if the value of the electricity consumed is enormous.  We must look closely at how the utility credits the exported energy from the PV system. Under NEM-2 (Net Energy Metering laws currently in effect until April 13, 2023) PV exported back to the grid is credited at the retail rate at that time. Its called time of use metering (TOU).  Under new laws coming into effect April 14, 2023 NEM-3 means the utility will only credit you the wholesale rate for the excess electricity exported back to the grid at the time of use. The following chart is crucial in understanding the value of the electricity the heat pump consumes.   

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This chart represents SDGE's  TOU rates. All these crucial details vary case by case but this one example will allow us to analyze SPH (solar pool heating) vs PHHP (pool heating heat pump). Note that if your PV system is NEM-2 grandfathered, the value of that electricity used to power the heat pump is at least $0.20/kwhr. Under NEM-3 however the value of that electricity is more like $0.05/kwhr. That's all the utility will credit you for electricity exported to the grid that you are using to power the heat pump instead. Its very important to note that under either net metering law you don't get to collect low rate and export expensive electricity. That was NEM-1. Those days are long gone. Under NEM-3 there will be excess PV electricity available during the day when the heat pump is most effective  and the opportunity cost to sell to back to the grid in real time is low. However, the energy required to power just the heat pump will be much more than the entire PV system can deliver so you will be paying for a large portion of the electricity at the time of highest cost.

Under the new NEM-3 metering laws, the idea will be to incentivize PV owners to add batteries so daily solar gains can be consumed when needed throughout the day. California already has enough PV with it's many utility scale projects and individual rooftop home systems combined. The idea of the new regulation is to try to reduce the exported electricity to the grid when the grid has no need for it.  If you want to look forward to NEM-3, PV systems with batteries, then you're going to be tempted to increase the size of the PV system so you have enough daily juice to run the heat pump. You can't realistically do that but if that worked out for you somehow, how much space would the extra PV require compared to solar pool heating? Which is the better use of space?  A 400 sq ft uncovered pool needs 400 sq ft of total roof area for SPH facing somewhere between wsw and sse.  A PV system to power a heat pump needs to be much larger than 7.3kw and 7.3kw takes up about 438 sq ft of rectangular area and that'll only run the heat pump the equivalent of 5 or 6 hours a day which is only enough for a small pool. The cost per area on the roof is much much lower for solar pool heating and we can use up much more of the available space due to our flexible fitted designs.

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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