We appreciate the time and effort Vaughan Woodruff took to assess and then write a thoughtful analysis of much of our paper (See RESOURCES for original study .pdf and this response). He accepted our conclusion that the evacuated tube domestic hot water system on the McDaniel home in Oberlin, OH, was not the most cost effective choice.
He did not agree that our study provided sufficient data and analysis to apply our conclusion to other places with climates similar to Oberlin’s (i.e., much of the northeastern US) for two major reasons. First, the small amount of hot water used made the results not applicable to the average US home. Second, the evacuated tube system (ET) appeared to be poorly designed, thereby giving suboptimal performance. Even if these criticisms hold, the physics of heating hot water and the data presented indicate that the fundamental conclusion of the paper still stands—evacuated tube hot water systems are not cost effective for domestic hot water heating in Oberlin-like climates.
We accept that by the use of water-conserving technologies and behaviors the McDaniel home achieved a hot water use of about a third of that used by the average two person home (annual use of 3,000 verses 9,000 gallons). Although the best way to reduce resource and energy use is not to use them (conservation), we recognized in the paper that most homes do use more hot water than the McDaniel home. Although we discussed the use of more water (2x, 4x, 8x), Woodruff did not comment directly on these data but rather cited other data to establish that the McDaniel water use was unusually small.
If the amount used was that of an average 2 person home (3x), the cost for heating water with the McDaniel electric on-demand system alone would be about $110. If 8x, the cost would be about $290 (8x = 24,000 gallons, an amount about 6,000 gallons over the average annual hot water use for a family of 4). If we assume that 50% of the heating energy came from the sun via the ET system, the cost savings would be about $65 (3x) and $145 (8x), respectively. After subsidies, the McDaniel ET system cost $4,589 giving pay back times of 71 years (3x) and 32 years (8x). In Oberlin, OH, electricity cost $0.10/kWh.
As we stated in the paper, the more expensive energy is, the quicker the payback. In the northeastern US many homes pay more for electricity than in Oberlin, some as high as $0.20/kWh. At this higher rate, paybacks times would be 35 (3x) and 16 years (8x). Yes, the higher the energy cost, the more economically viable an ET becomes. Of course, as people apply more hot water conservation measures, the less viable the ET becomes.
Woodruff hypothesized that the system was poorly designed because the Bosch RP17PT on demand, hot water heater was not intended to be used with preheated water, and the appropriate on demand heater would likely use less energy. He also hypothesized that the mixing valve installed on the hot water outlet from the preheat tank could have been set to a temperature that unnecessarily added cold water to the water fed to the on-demand heater, again causing the use of more energy by the on-demand heater. The mixing valve was set to 120° F (range 80° F to 140° F) but, as Woodruff noted, the accuracy of this setting was not confirmed. Because the system was removed in September 2010, we cannot test these hypotheses. In addition, Woodruff indicated that the pumping design in the ET system was suboptimal. This could very well be true. However, we sent our results to the supplier who sent them to the manufacturer. The manufacturer responded with a number of things to check that the supplier ruled out as causes for the poorer than expected performance.
In the paper we recommended an on demand hot water system as the optimal choice for domestic hot water in climates like those of Oberlin, OH. The McDaniel on demand heater cost $499, not including cost of wiring. For installation in an existing house the cost would be higher. With the on demand alone, the McDaniel annual energy used for 3,000 gallons of hot water is about 360 kWh or $36, if purchased (McDaniel electricity is provided by PV panels). It is difficult to know what hot water system to compare this to in order to estimate savings and payback time. However, because the on demand system alone cost one tenth that of the combined system ($499 vs $5,088), the pay back time would be 10x longer for the combined system than for the on demand heater alone, because of the cost of the ET component. This economic fact alone makes the ET an inappropriate choice for climates like that of Oberlin, OH.
Woodruff also suggests that the ET system was too large for the amount of hot water used by the McDaniel home (i.e. too many evacuated tubes and too large a preheat tank). At first this appears to be a valid criticism; however, many factors make this a difficult assertion to confirm: preheat-tank temperature (heat) loss dependent upon tank size and insulation (not stated in paper but the McDaniel preheat tank had R-21 of insulation added to the tank that cut heat loss in half), when hot water is used, when and how long the sun shines related to when hot water is used, and seasonal variations in amount of solar heating, to name a few. I suspect that the size of the system was not the mismatch Woodruff believes it was.
As we discussed in the paper, ET systems are excellent at acquiring heat energy. Even in climates like that of Oberlin that get about 50% of possible sunshine (mostly in the warmer months), ET systems are appropriate for restaurants, laundries, hospitals, hotels, dormitories, and other situations that have sufficient hot water demand to use the hot water as it is heated.
Customers have to trust that suppliers and manufactures provide quality equipment that is well designed to do what it is advertized to do until proven otherwise. Very few people know how many gallons of hot water they use or the amount of energy consumed by their hot water systems, because these uses are not metered separately. Nor do they know the fraction of the energy used to heat their water that is actually in the hot water used. As a result, customers are totally in the dark about the performance of their hot water systems. Our study of the McDaniel combined hot water system has provided the data and analyses to establish that ET systems are not the optimal choice for heating domestic hot water in climates like those in Oberlin, OH, including most of the northeastern US.
We can go round and round on the technical details of the McDaniel combined system, or other hot water systems, but the basic physics of heating water dictates the optimal means for heating water. Solar hot water makes sense economically when the transfer of heat to water is efficient, the water is used when heated, and the cost of the devices to transfer solar heat is not substantially more than heating water at the time of use with electric or combustion heat. Our study of hot water heating in the McDaniel home established that the physics of heating hot water in climates like that of Oberlin dictate that ET systems are not optimal compared to on demand heaters. ET systems simply cost too much at current energy prices.
Carl N. McDaniel
Environmental Studies Department, Oberlin College
David N. Borton
Solar Age Technologies