Evacuated Tube Solar Hot Water Systems Are Not Energy Efficient or Cost Effective for Domestic Hot Water Heating in the Northeastern U.S.

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1 Evacuated Tube Solar Hot Water Systems Are Not Energy Efficient or Cost Effective for Domestic Hot Water Heating in the Northeastern U.S. Climate Carl N. McDaniel¹ and David N. Borton² ¹Environmental Studies Department, Oberlin College, Oberlin, OH 44074, USA ²Solar Age Technologies, Troy, NY 12180, USA ABSTRACT Domestic hot water was preheated with an AP- 30 evacuated tube system and preheated water was stored in 80 gallon tank before being warmed to 111 F, if needed, by a Bosch RP17PT on demand electric heater. Over a 6 month period we measured: 1) incoming water temperature, 2) the kwh to pump fluids in evacuated tube system, 3) the kwh to heat water by on demand heater, and 4) the gallons of hot water used. These data were analyzed to establish that sunshine provided ~8% of the energy to heat the hot water used annually. Less than 1% of the annual solar heat energy harvested by evacuated tube system was in hot water used: ~9,000,000 BTUs harvested; ~81,000 BTUs used. This 2 person household annually uses ~3,000 gallons hot water while the average 2 person household uses 9,000 gallons. Even at this higher hot water consumption only a small fraction of the harvested solar BTUs would be used. The evacuated tube system cost, $6,589; the on demand heater cost, $499, not including cost of wiring. With the evacuated tube system saving 30 kwh or ~$3.00/year, it was a very poor investment economically and equally ineffective for reducing carbon emissions. The evacuated tube system was removed after two years. Data collected over a 3 month period indicated that heating water to 111 F with the on demand heater used 0.12 kwh/gallon. With an annual use of 3,000 gallons, the electrical energy required to warm water with the on demand heater is estimated to be 360 kwh annually or ~$ INTRODUCTION Solar heating of domestic hot water has had a long history (Butti and Perlin, 1980). When bathing became common in the 1800s, hot water for baths was heated with wood or gas, usually on a stove or with pipes in the fire box. The first solar hot water heaters were metal tanks painted black and positioned to be in full sun. They worked, but temperature control was minimal. In 1891, Clarence M. Kemp patented the first commercial solar water heater called the Climax. The Climax came in several sizes, the largest being 700 gallons that sold for $380. Although expensive, the energy source for heating water was free. Soon after the turn of the last century, William J. Bailey developed the Day and Night hot water heater (Butti and Perlin, 1980). Instead of leaving the warmed water in a tank on the roof, water was solar heated in a small tank outside the house and then flowed to a large storage tank inside the house that was insulated thereby keeping the water warm through the night losing about 1º F per hour. The small tank was placed lower than the hot water storage tank so that no pumping was required because the less dense hot water moved to the top of the storage tank while the denser cold water sank to the smaller tank where it was heated. The Day and Night solar hot water system became popular with several thousand installed by the early 1920s in California and other warm, sunny climates. From the twenties to the mid fifties, domestic solar hot water heating systems continued to improve (Butti and Perlin, 1980). However, over the same period, increased competition from inexpensive energy supplied by natural gas and electricity made solar hot water systems too expensive and few were sold. For the remainder of the century, the solar hot water industry persisted as a fringe market that is now beginning to gain attention because of the increasing cost of energy and the concern over heat trapping gases that are released with the burning of fossil fuels. 1

2 In 2007, Carl and Mary McDaniel elected to equip their new home, Trail Magic, with an evacuated tube hot water system that fed an electric on demand, hot water heater. At the time they believed it would be an efficient, cost effective system with sunlight providing most of the energy used to heat their hot water. To their and others surprise, however, the data collected and analyses made established that evacuated tube hot water systems are inappropriate for heating domestic hot water in most situations but especially in climates similar that of Oberlin, OH. MATERIALS AND METHODS Data collection on and analysis of the performance of the AP-30 evacuated-tube, solar hot water system (ET) and the Bosch RP17PT on demand electric heater (OD) ET was installed on a directly south facing roof with a 33º pitch (Figure 1). The ET, preheated water was stored in a Bradford White Corporation, 80 gallon electric hot water tank (M280R6DS-1NCWW; not wired to heat water) before being warmed to 111 F, if needed, by the OD (Figure 2). Figure 1. AP-30 evacuated tubes being removed from roof in September Figure 2. Well insulated 80 gallon storage tank, AP- 30 pumps and heat exchanger, and Bosch on demand electric hot water heater. The amount of solar energy supplied by the ET to the hot water used was calculated with the following data and procedures. 1. The temperature of the water coming into the house was measured with a thermometer on the 15 th and last day of each month over a 6 month and 3 month period. 2. The hot water temperature at the faucet was measured with a thermometer five times over a 6 month period and twice over a second 3 month period. 3. The amount of hot water used was measured using a water meter on the water line feeding the hot water storage tank. 4. The electrical energy (kilowatt hours [kwh]) to heat water by the OD was measured with The Energy Detective that recorded current flow thorough the two, 240 volt circuits that feed the OD. 5. The electrical energy used to pump glycol from the roof and water from the storage 2

3 tank through the heat exchanger was measured with a Kill-a-Watt meter into which the pumps were plugged. 6. The total amount of energy to heat the hot water used was measured in British Thermal Units (a BTU is the amount of heat needed to raise one pound of water 1 F) and calculated by multiplying the gallons of water used temperature difference between the incoming water and faucet water 8.34 BTUs/gallon F. 7. The net amount of energy provided by sunshine that was in the hot water used equals the total BTUs needed to heat water to faucet temperature, minus the BTUs of electricity to pump fluids in ET, and minus the BTUs of electricity to heat the water with the OD. Conversion: 1 kwh = 3412 BTUs. 8. Net percent of energy from sunshine equals (net solar BTUs total BTUs) 100. Calculation of annual BTUs acquired by the ET The temperature of the preheated water in the storage tank is given on the ET pump station display and was used to measure energy gain and loss as follows. The energy gain in BTUs was calculated as follows: 8.34 BTUs/gallon F 80 gallons # of degrees risen. The temperature loss over the period water was heated was calculated as follows. The temperature loss at night was measured for different tank temperatures at 10 degree intervals (e.g. for tank temperatures between 110 F and 120 F, the temperature loss was 1.5 F/hour; between 90 F and 100 F, the temperature loss was 0.63 F/hour). The energy loss during the period of heating tank water by ET given in BTUs was calculated as follows: 8.34 BTUs/gallon F 80 gallons # of hours heat was lost average temperature loss/hour for the temperatures of the tank over the loss period (e.g. if temperature was in the 91 F to 99 F range, the temperature loss factor would be 0.63 F/hour). A 3.12 kw photovoltaic system (PV) is mounted on the same south facing roof as the ET. On fifteen days over the period from 24 February 2009 to 17 June 2009, the kwh produced by the PV and the temperature rise in the ET preheat tank were used to estimate the annual BTUs acquired by the ET as follows. The BTUs acquired by the ET and retained in the storage tank water were calculated from the temperature rise recorded. The BTUs lost from the preheat tank were calculated as described above and added to the BTUs acquired and retained. The total BTUs acquired by ET were divided by the PV kwh produced to give the ET BTUs/kWh. The ET BTUs/kWh were multiplied by 3192 kwh (kwh produced by the PV in 2009) to give a rough estimate of the annual BTUs acquired by the ET. Assumptions We assume that the heating of water by electricity in OD is 100% efficient. This is not true, but if water were heated constantly by OD, the heat transfer efficiency would be close to 100%. We also assume that no heat is lost in the pipes between the OD and faucet or shower head. This is not true, but it has minimal influence on the conclusions here. Heat loss in pipes, however, does increase the amount of hot water used. That is, the volume of water in the pipe must flow out before any hot water is delivered and then the pipe needs to be warmed to provide hot water at a constant temperature. We assume that the incoming water temperature is the temperature of the water in the storage tank that is then heated by solar input or by OD electricity. This is not true because the storage tank is in a room having a temperature range of a low of ~55 F in the winter and a high of over 70 F in the summer that warms the water in the tank when tank water is cooler than room temperature. When we scaled up water use, we assumed that the energy used by OD heater scaled up linearly. This is not true because the temperature of water in the storage tank is reduced when hot water is used and cold make-up water enters the tank thereby requiring more energy to heat it in the OD. Additionally, the temperature of water in the storage tank changes in a complex way if the sun is shining and hot water is used. We cannot 3

4 estimate the influence of these variables other than to say that they would probably increase the amount of energy provided by OD while reducing the solar component. Although not relevant for the analyses made here, most of the heat acquired by the ET is lost to the air around the storage tank thereby heating the house. In winter this is a useful input; in the summer, it is unwanted. We used raw data with seven significant figures. Our confidence in the data justifies two significant figures. That is, we are confident in our kwh numbers to two significant figures (5.5 or 120 or 3,200 but not 5.54 or or 3,24l.56). RESULTS Solar energy component of the energy used to heat hot water with combined ET and OD systems The average annual incoming water temperature was calculated to be 55 F with a high of 67 F in August 2009 and a low of 42 F in February With the OD set at its middle temperature setting, the average faucet temperature was 111 F during the 6 month and 3 month experimental periods with a high of 112 F and low of 110 F. Based on 27 months data for hot water used, the annual use was 2,933 gallons, rounded off to a use of 3,000 gallons/year. From 14 August 2009 to 8 November 2009 and from 1 December 2009 to 13 February 2010, we recorded the kwh to pump fluids in the ET, the kwh used by the OD, the gallons of hot water used, and the incoming water temperature. We considered 14 August to 30 September to represent summer conditions, 1 October to 8 November to represent fall and spring conditions, and 1 December to 13 February to represent winter conditions. In summer conditions to heat 479 gallons of water from 67 F to 111 F theoretically requires 175,774 BTUs if heated solely by the OD (479 gallons 44 F 8.34 BTU/gallon F). Empirically it required 34,120 BTUs (10 kwh) for OD, 139,892 BTUs (41 kwh) to pump fluids in the ET, and 1,762 BTUs of solar heat (175,774 BTUs [theoretical] 174,012 BTUs [electricity] = 1,762 BTUs [solar]). Therefore in summer it takes 367 BTUs/gallon of which 4 BTUs are solar or ~1% solar heat in summer hot water. In fall and spring conditions to heat 310 gallons of water from 59 F to 111 F theoretically requires 134,441 BTUs if heated solely by the OD (310 gallons 52 F 8.34 BTU/gallon F). Empirically it required 47,768 BTUs (14 kwh) for OD, 78,476 BTUs (23 kwh) to pump fluids in the ET system, and 8,197 BTUs of solar heat (134,441 BTUs [theoretical] 126,244 BTUs [electricity] = 8,197 BTUs [solar]). Therefore in fall and spring it takes 434 BUTs/gallon of which 27 BTUs are solar or ~6% solar heat in fall and spring hot water. In winter conditions to heat 318 gallons of water from 45 F to 111 F theoretically requires 175,040 BTUs if heated solely by the OD (318 gallons 66 F 8.34 BTU/gallon F). Empirically it required 75,064 BTUs (22 kwh) for OD, 75,064 BTUs (22 kwh) to pump fluids in the ET system, and 24,912 BTUs of solar heat (175,040 BTUs [theoretical] 150,128 BTUs [electricity] = 24,912 [solar]). Therefore in winter it takes 550 BUTs/gallon of which 78 BTUs are solar or ~14% solar heat in winter hot water. These calculations indicate that annually it takes on average 447 BTUs/gallon to heat hot water in the combined system ([367 BTUs BTU BTU BTU] 4) of which 34 BTUs/gallon come from solar heat ([ ] 4) or ~8% of the heat in the hot water used comes from the sun (Table 1). Table 1: Energy inputs measured in BTUs in each gallon of hot water used. Season Total On Demand Electricity Evacuated Tube Electrici- Solar % Solar ty Summer Fall & Spring Winter Annual With an annual use of 3,000 gallons, the value of the solar component at $0.10/kWh was $3.00 ([3000 gallons 34 BTUs/gallon] 3412 BTUs/kWh = 29.9 kwh; 30 kwh $0.10/kWh = $3.00). Energy required to heat hot water with the OD 4

5 The ET system was removed in September 2010 because it was energy and cost inefficient at providing hot water. Water was then heated solely by OD. Theoretically the electrical energy required to heat 3,000 gallons of water from 55 F to 111 F with OD is 410 kwh (3000 gallons/year 56ºF 8.34 BTUs/gallon F = 1,401,120 BTUs/year; 1,401,120 BTUs/year 3,412 BTUs/kWh = 410 kwh/year) or 0.14 kwh/gallon (410 kwh 3,000 gallons). Thus, the estimated annual cost for 3,000 gallons of hot water at $0.10/kWh would be $41.00 (410 kwh/year $0.10/kWh). The electricity actually used by the OD was measured from October 26, 2010 to February 15, The interval from October 26, 2010 through November 31, 2010 was used to calculate the annual energy required per gallon of hot water used because during that period the incoming water averaged 55 F, the annual average temperature of incoming water. During this period 335 gallons of hot water were used and the OD used 40 kwh. Thus, over the year it took an average of 0.12 kwh/gallon of hot water used (40 kwh 335 gallons). The actual annual energy required for heating 3,000 gallons of water with the OD would therefore be 358 kwh (0.12 kwh/gallon 3,000 gallons/year) or $36.00 (358 kwh/year $0.10/kWh). This reduced electrical energy use compared to the theoretical energy use (0.12 kwh/gallon verses 0.14 kwh/gallon) resulted because the water in the storage tank was heated ~5 F by room heat before being heated in OD. Annual BTUs acquired by ET and BTUs in hot water used The average annual BTUs acquired by ET were 8,851,416 with a range of 6,000,960 (March 2, 2009) to 12,774,384 (June 6, 2009). Uncontrollable variables, known and unknown, account for the wide range making this number a rough approximation of the total energy acquired by ET. DISCUSSION Trail Magic was designed and equipped to conserve hot water. Low flow shower heads deliver 1.5 gallons/minute and bathroom faucets deliver 0.5 gallons/minute. The clothes washer is a front loading machine that uses 2 gallons of hot water per regular cycle even when on cold water setting, because the machine was engineered to use some hot water even on the cold water setting. The dishwasher used 5 gallons of hot water per regular cycle. Residents take water efficient showers that consume between 4 and 8 gallons per shower. Guests may or may not take water efficient showers. These design features and resident behaviors have led to an annual, hot water use of 3,000 gallons over the first 2.5 years in the house. This compares with the annual average hot water use of ~9,000 gallons by 2 person U.S. homes (DeOreo and Mayer, 2001). Design features and resident behavior have conserved hot water to the extent that the expense of the ET was not justified economically with a pay back time of more than a 1,000 years ($6,589 [cost] - $2,000 [federal tax credit] = $4,589; $4,589 $3.00/year [savings for solar energy] = 1,530 years). Even if all the energy to heat water came from solar, the payback time would be 127 years ($4,589 $36 [cost of 360 kwh/year]). Carbon dioxide payback time is also likely to be long with a savings of 60 pounds/year (in Ohio about 2 pounds of carbon dioxide are released for each kwh produced), but an estimate cannot be made because the embodied energy in the evacuated tube system is not known. If more hot water is used, would the solar hot water be cost effective economically and environmentally? If 2x, 4x, and 8x more hot water are used, the total cost of the kwh for hot water would be $72, $144, and $288, respectively, with savings from the solar component being difficult to determine by likely no more than half the cost even for the highest usage. The 8x amount of hot water is 24,000 gallons or about 6,000 gallons more than the average use rate for four people. The payback time would be ~30 years if half the energy came from solar ($4,589 $144). An alternative way to evaluate payback times is by assessing the dollar value of the annual ~9 5

6 million BTUs acquired by the ET: ~$265 if compared to equivalent BTUs in electricity or 2,650 kwh (9,000,000 BTUs 3412 BTUs/kWh) at $0.10/kWh. The payback time would be ~17 years. At this 100% use level, carbon dioxide payback time would be maximized but we do not have the information to estimate this payback time. To use all the solar heat energy acquired by the ET, the residents of Trail Magic would have to take 8 hours of showers every day, 365 days a year. That is, in Table 1 we estimated there are 34 BTUs of solar energy in each gallon of hot water used; therefore, nine million BTUs 34 BTUs/gallon = 264,705 gallons; 264,705 gallons 1.5 gallons/min [shower flow rate] = 176,470 min; 176,470 min 60 min/hour = 2,941 hours; 2,941 hours 365 days = 8 hours/day. This calculation is imprecise but indicates the substantial amount of heat acquired by the ET and how little of it an average residence uses. If the ET were employed in areas with a higher percentage of possible sunshine, the BTUs acquired would increase making payback time less. Trail Magic in Oberlin, OH, receives ~49% of possible sunshine, but if it were in Yuma, AZ, that receives the highest percentage in the U.S. at 90% (NOAA, 2004), the acquired BTUs would likely double to perhaps 20 million with a payback time of less than 10 years. The above payback calculations indicate that an ET is best employed in situations where all the solar heated water can be used as it is heated. Restaurants, laundries, hospitals, hotels, and dormitories are more suitable locations than single family residences, even in areas with high percentages of sunshine. The ET system in northeastern U.S. climates has the additional negative attribute of acquiring most of its energy in the warm months of May through September, thereby heating the house at times when additional heat is not usually wanted. In climates with substantial winter sunshine, however, heat not used for hot water would beneficially warm the house on cold days. With the information obtained in this study on the hot water systems at Trail Magic, it is clear that an ET is not cost effective under present economic conditions for residential hot water in most situations and especially so in northeastern U.S. climates. The OD is cost effective compared to the standard gas or electric hot water heater because in these hot water tank systems at least 50% of the energy used to heat water is dissipated into the area surrounding the tank. For example, the annual cost of hot water in the McDaniels previous home with gas heated water was $85 for 72 therms of gas ($1.20/therm) verses $36 to heat solely via the OD at Trail Magic. Although the McDaniels probably used more hot water in their previous home, it was unlikely 25% more, making the on demand heater perhaps twice as cost effective as a gas or an electric storage tank system. The data collected and analyses made indicate that the most efficient hot water system for a single family home would appear to be an OD fed from a large, uninsulated, storage tank (80 gallons or more) that can equilibrate to room temperature. As we observed, water in the storage tank is slowly heated by room air at a cost less than that of instant electric heating. ACKNOWLEGEMENT We thank Mike Foraker, President of The Energy Factory, for technical assistance and for his open minded approach in supporting the assessment of the performance of the AP-30 evacuated tube solar hot water system that is firm installed and then removed at Trail Magic. REFERENCES Ken Butti and John Perlin, A Golden Thread: 2500 Years of Solar Architecture and Technology (Palo Alto, CA: Cheshire Books, 1980). NOAA 2004, /pctposrank.txt (accessed June 2, 2011). William B. DeOreo and Peter W. Mayer, The end uses of hot water in single family homes from flow trace analysis, Boulder, CO: Aquacraft, Inc. (2001) 6