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Solar Hot Water: Which Is Better PV or Thermal?

Solar thermal water heating is a temperamental thing. Water weighs a lot, it expands when it freezes, and it can cause scaling damage to pipes when it boils. Solar thermal systems are wonderfully efficient, and some systems work just fine for decades, but even these need regular inspection. When a solar thermal system fails, however, it sets about destroying itself, and it has been clear for some time that solar thermal water heating is not the way of the future except for very low-end heat usage, like swimming pools.

For a long time now, the wisdom has been that the relative efficiency advantage of solar thermal technology for water-heating more than outweighs the convenience of electric water heating. The ability of solar thermal to collect more energy per square foot means that a solar electric system powering a conventional electric water heater alone will never compete with a solar thermal system. 

Recently, however, reductions in solar electric (PV) costs and maturation of air-to-water heat pump technology have provided a new model: solar-electric assisted heat pump water heating (HPWH). HPWH comes with fewer drawbacks than solar thermal, with a smaller price tag for residential applications. 

The information below assumes the use of a heat pump water heater with an efficiency factor (EF of 2.5) and an 1,800 kWh per year rating, with 1 to 1.3 kW of grid-tied PV added to existing installation or system in a region where the PV produces at least 1,400 kWh/kW/year. 

PV Advantages

Lower upfront cost: Given that lower-cost open systems have proven to be unsuitable for domestic water heating, the installed cost of solar thermal should be based upon a closed loop (glycol or drainback), two-tank (or storage plus tankless) system, fully installed. The average price for such a system, designed for a family of four, is between $7,000 and $10,000 before incentives. The PV powered heat pump water heater will cost between $1,000 and $2,000 for the heat pump plus labor and between $3,500 and $6,000 for the additional PV (to an existing grid-tied system), thus a total installed cost of between $5,000 and $8,500 before incentives. 

Easier to install: Replacing a water heater with another single tank and adding three to five additional modules to a PV system is far easier than replacing a single tank with two tanks and piping heat transfer fluid to heavy rooftop panels that must be pressure tested and charged after installation. This results in fewer opportunities for installer error.

Uses less space: To avoid having the solar thermal system compete with the backup source (which limits the solar fraction to about 60%), two tanks are required: one for the backup, and one for the solar. It is possible to save space, at great expense, with the use of a tankless heater as long as the tankless heater can modulate the heat flow down to a very low point while being able to also meet maximum demand. 

Needs no maintenance: The Achilles Heel of solar thermal is that if the system stops working, it does not just fail to produce energy: it sets about its own self-destruction. Without flow the panels can freeze or stagnate and overheat (see below). The electronic differential controller and circulator pump(s) must be inspected yearly to assure they are functioning properly and that no scale or corrosion have begun that will lead to system failure. The piping should also be checked, especially for drainback systems in older buildings that may settle over time and trap fluid in the lines. These annual inspections must be performed by a professional, and will cost half of the yearly gas savings.

Cannot freeze: Since a solar thermal panel can freeze at temperatures as high as 42ºF, freeze protection is required throughout the mainland US for solar thermal systems. With the exception of drainback systems, freeze protection systems are "active". This means they require a device to operate in response to low temperature. As a consequence, and since they are rarely required to function, freeze protection failures are both common and catastrophic, resulting in thousands of dollars of damage to the collector array.

Cannot overheat: Overheating is a frequently overlooked problem with solar thermal systems. There is approximately twice as much solar energy delivered in July as in January. Thus, any system that will make a significant difference in hot water cost in January will over-perform in July. This results in periods of stagnation where there is no use for the solar heat and no flow through the panel(s). Under this condition, the panels will heat to around 400ºF inside. This can result in damage and accelerates the deterioration of the collector parts. There are radiator systems that have been added to panels to mitigate this effect, but there are no solid data on how much radiator is required to cool a stagnant collector on a hot day.

No scale build up: Scale is the #1 enemy of water heaters of any type. Heat makes dissolved solids precipitate from water where they collect on the hot surface. Even with the use of a transfer fluid on the collector side, scale can be a problem with the heat exchanger by clogging the tubes the water flows through to gain heat. The lower temperatures used to heat water with a heat pump reduces the tendency of scale to build up in the tank.

100 percent solar fraction attainable: Due to the vagaries of weather and the impracticality of storing large volumes of hot water, no solar thermal system that offers 100 percent reliability can have a 100 percent solar fraction. The systems most highly rated under the SRCC OG300 protocol have a 90 percent solar fraction. Using grid-tied PV as the solar source for the heat pump water heater allows the system to "store" power in the grid for use up to one year later. The price comparison above is based upon a thermal system with an 80 percent solar fraction versus a 100 percent PV offset for the water heating.

Grid demand management: Although heat pump water heating adds a load to the grid when used to replace a gas or propane unit, the PV adds power to the grid during peak daylight hours where it is most likely to be needed by the community. Most household hot water is used early in the morning and evening when there is less community-wide electric demand. If the utility elects to use this advantage, it could also add the ability to overheat the water heater through the smart meter when excess electricity is available on the grid. Used in conduction with a mixing valve to protect the house from scalding water, it effectively "banks" hot water and can delay the need for the heat pump to turn on.

No CO2 emissions: Any use of natural gas or propane, regardless of how efficient or cheap, results in the addition of CO2 to the atmosphere which is the #1 risk factor facing civilization today. A heat pump water heater that is 100 percent powered (or offset) by PV makes no contribution to that problem.


Net grid efficiency v. direct gas use: The standard presumption when comparing gas use with electric use is that, after accounting for conversion and transmission losses, it takes three units of fossil fuel energy (gas, oil, coal) to deliver one unit of electric energy. Thus the rationale that if gas can be delivered to the point of use, it is more efficient to use the gas than to use electricity.Since most fossil-fueled water heaters are only about 60 percent efficient, this effect is only half as significant as it appears. Additionally, fossil-fueled water heaters fail to take advantage of Renewable Portfolio Standards that further reduce the ratio of gas used to electricity delivered. 

Warm air required: The efficiency of the heat pump water heater depends upon the available heat source which is usually the air in the space in which the heater is placed. Installed in unheated spaces in temperate climates, this presents no problem. However, if the water heater space is heated or drops below 55º-60ºF much of the year, the backup element will be needed and efficiency will suffer. Conversely, the heat pump water heater will cool and dehumidify the space in which it is located. This may be a desirable feature. 

Newer on the market: Though air-to-water heat pump water heating uses only tried-and-true concepts, household HPWH has only had about twenty years of development on the consumer market: long enough to be confident in its efficiency and ease, but not long enough to be widespread. While there are about five hundred solar thermal models and six hundred tankless ("instant") water heaters recognized by the DOE's Energy Star system, there are currently only 23 recognized HPWH models.

For what it was, solar thermal technology represented an improvement. It does still have some legitimate applications, even. However, household-level solar water heating comes with so many unnecessary drawbacks that it is clear the future lies in another direction. Solar photovoltaic is a highly-effective source for a heat-pump water-heating system. Soon, that water-to-water heat pumps may be available on the market, but today's air-to-water systems are the optimal selection for many households, depending on climate and configuration.

Lead image: PV panel and pool via Shutterstock

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