Recently Renewable Energy World published an article entitled "Solar Hot Water: Which is Better PV or Thermal?" The piece argues that the reduced cost of solar photovoltaics (PV), in combination with certain advantages of air source heat pump water heaters, have made heating water with medium temperature solar collectors obsolete.
Find the original article here.
By oversimplifying the topic, such articles serve to confuse the general public and draw attention away from those applications where solar water heating (SWH) is clearly a better option than PV. This particular example overstates the disadvantages of SWH and makes some overreaching assumptions that are not universally applicable.
Overstating the Disadvantages of SWH
- Overheating. There have been significant advances in understanding the behavior of SWH systems during stagnation (no-flow) conditions that have influenced the design and installation of systems. As a result, there are ways to mitigate overheating that are far simpler than using radiators and will not lead to the glycol deterioration described in the article.
- Freezing. Properly installed SWH systems are not susceptible to freezing. Direct systems — those that directly heat potable water with their collectors — are subject to freezing, but will not “set about destroying [themselves].” Freezing is easily avoided by selecting the right type of system for the application. For most, if not all, of the continental U.S., this means using a drainback or antifreeze system.
- Maintenance. It is a dramatic overstatement to claim that yearly professional inspections are required for a SWH system. Like PV, SWH systems can be monitored remotely and system operation can be verified fairly easily by the homeowner or facilities manager. It would have been more accurate to state that the maintenance schedule of a well-designed and well-installed SWH system is nearly equivalent to the heat pump water heater technology promoted in the article.
- More space. There are a number of SWH systems that utilize a single tank for water heating and utilize controls to allow for solar fractions that are roughly equivalent to their two-tank counterparts. This is certainly a challenge with natural gas, but using solar to offset natural gas in residential applications — whether with SWH or PV — is currently an economic challenge due to suppressed prices for natural gas.
- Free storage. The fundamental policy device that has driven the implementation of grid-connected PV is net metering. In many states, these policies essentially allow for highly efficient “storage” of electricity. As a result, this allows grid connected PV systems to achieve something that has been outside of the reach of SWH — long-term solar storage. Through net metering agreements, solar electricity produced in July can be used in January. Small scale SWH will never be able to reach this storage efficiency. It is worth noting that not all jurisdictions have net metering, and these arrangements are very dependent upon policy. For example, some customers in Vermont are no longer able to net meter now that their electric utility has satisfied their solar quota. Arizona Public Service (APS) in Phoenix has pursued policy that would restrict the value of exported, net-metered electricity. If this is successful, electrical “storage” that was assumed to be near 100 percent efficient suddenly becomes less than 50 percent efficient. As distributed energy production becomes more prevalent, we will see demand for distributed storage as well. SWH systems are particularly effective at this.
- Roof space. With current technology, it typically takes nearly four times the roof area for PV to produce the equivalent energy as a SWH system. There are plenty of homes without adequate roof space for a robust PV system, and in certain areas of the country available roof space is further limited by shading from adjacent trees and buildings. SWH is less susceptible to shading than PV, thus it is more adaptable under less-than-ideal circumstances.
- Assumed storage requirements. While many SWH systems are analogous to off-grid PV systems (another application where SWH has a favorable advantage) due to the requirement for energy storage, there are a number of applications where SWH behaves very similarly to a grid-interactive PV system. In facilities that use large volumes of hot water, heat storage may not be a huge concern as the heat is used as soon as it is produced. Due to the significant efficiency advantage of SWH collectors versus PV cells, these applications are highly favorable for SWH.
- Water heating demand. Another underlying assumption is that the energy required for water heating will remain fairly static. Increasingly, jurisdictions are enacting policies that will prove otherwise. Due to an increased body of understanding of legionella, local jurisdictions are beginning to require that water heaters be maintained at 140°F to reduce or eliminate the opportunity for the bacteria to grow in water heaters. At 140°F, the efficiency of heat pump water heaters is significantly less than under the assumed conditions.
- Utility pressure. Many investors recognize the threat that distributed electrical generation presents to their business models unless they take steps to adapt. Since PV has the ability to be implemented in utility-scale installations, there will be significant effort by the utilities in the coming years to shift their centralized fossil fuel production model to one that is similar in its infrastructure but that utilizes PV for its production. As such, PV is likely to see more political pressure and competition in the coming years. Since SWH is by necessity an exclusively distributed energy source, it is less likely to be subjected to the policy threats facing PV.
Heating is much more nuanced than electricity supply, and the comparative lack of standardization of SWH compared to PV has been a challenge to its widespread implementation. Neither technology is a panacea — instead the goal of the industry should be to identify the application for which each technology is best suited. Articles that do otherwise are ultimately a disservice to our industry and our customers.
There is clearly a changing tide in our industry that has been driven by significant price drops in PV technology, favorable policy, and the implementation of financing models that have promoted the accessibility of solar technology for people across the economic spectrum. PV is more suitable for a broader range of residential applications than it was three years ago. That being said, it is quite premature to proclaim the death of the solar heating industry.
Articles that make sweeping claims to conclude the superiority of PV over SWH draw attention away from the viability of specific technologies for specific applications. Rather than pitting one sector of the solar industry against another, we should concentrate on educating the public on the benefits of both.
Lead image: Solar hot water system via Shutterstock