From Today's Air Conditioning, June, 1999
Two homes, one standard (Control) and one a super-efficient photovoltaic residence (PVRES), are undergoing side-by-side testing and evaluation in Lakeland, Florida, to determine the extent to which contemporary energy efficiency technologies can reduce the demand for electricity in Florida homes. The measured results are very encouraging.
The average measured air conditioning load profile for the two test homes during June 1998 shows a 70 percent reduction in air-conditioning energy use for the PVRES home during the hottest month on record in Lakeland, Florida. All of these air conditioning savings were achieved through comprehensive energy-efficiency improvements that can be used in any home today. The PV power production capability of this home reduces its net power use to very near zero.
One of the most important aspects of the success of the PVRES home, is the ability to take advantage of all the features designed to reduce cooling loads. We used RHVAC (from Elite Software) to calculate the machine size for both the standard home and the control home in Lakeland, Florida. RHVAC uses Manual J for its calculations.
We entered all of the building features for either house in detail (walls, roof, glass, duct system etc.), but as a conservatism we chose an 95(F outdoor design temperature rather than the 91(F, suggested by Manual J along with a 75(F interior temperature. The Manual J calculations suggested a cooling system of 3.88 tons for the standard home (4 tons) and 1.73 ton (2 tons) for the PVRES house.
Although, the two ton system for such a large home (2,400 square feet) is highly unusual, Tim Rice of Ward's Air Conditioning in Lakeland has worked with FSEC and with Keith Ledford and John Price out of the Orlando and Tampa Trane offices to come up with a suitable system. In a related project, we have had a very good experience with Trane's XL 1400 series air conditioner. Consequently, we were able to select the TWY024A two-ton heat pump for the project.
We will use the TWE040E13 variable speed indoor air handler to provide optimum efficiency, humidity removal and quiet operation. The Seasonal Energy Efficiency Ratio (SEER) of the combination is 14.4 Btu/W; the analogous Heating Season Performance Factor (HSPF) is 8.5 Btu/W. For the standard home we will use a standard efficiency 4-ton Trane heat pump -- a TWR048C (SEER= 10.0 Btu/W; HSPF= 7.0).
After each air conditioner was successfully installed by the contractor (Ward's Air Conditioning in Lakeland), we performed one-time tests to establish the relative performance of the two units.
To perform the tests, we developed a procedure that can be used to measure the instantaneous efficiency of a cooling system. The procedure is entirely based on physical measurements. First, the dry coil air flow is determined by turning on the heat pump back-up resistance heat elements and measuring the temperature rise across the heating coil.
We used a portable Cooper Instrument Corp. (Middlefield, CT) SH66A multi-probe digital thermometer to take this measurement. By simultaneously measuring the element wattage (we turned off all non-AC breakers and measured this using the utility meter), the air flow cfm can be gauged through knowledge of air's specific heat and density. As a check, we used a Shortridge flow hood to verify the estimate obtained by the resistance heat method (one hint for anyone doing this: make sure that the thermometer after the coil is around the first bend in the duct work, or at the first register, or you will risk poorly mixed air in the air stream after the coil).
Secondly, the air conditioner is turned on to cooling and the temperature drop across the coil is measured. Within the tests, we used two temperature probes before and after the coil. One set of the probes recorded the dry bulb temperature. During cooling operation, the other two probes had saturated cotton shoe-laces inserted into the air stream to measure the wet bulb temperatures before and after the coil. This would allow determination of the coil latent performance by looking at the change in enthalpy across the evaporator coil.
As a check on this measurement, we also collected air conditioner condensate over a ten minute period once flow had begun.
Finally, an outdoor temperature is taken at the condenser inlet since the rated EER of units is typically based on this condition and that indoors. The entire procedure is described in detail on-line.
The product of the change in enthalpy across the evaporator coil times the measured air flow yields the total cooling. By measuring the power demand of the AC system, the system energy efficiency ratio (EER) is obtained.
The measured air flow for the four ton heat pump was 1555 cfm or about 390 cfm/ton. This is well within the tolerance established for the air conditioner (400 cfm/ton). A 16.5 degree temperature drop was measured across the coil (Treturn = 69.2o; Tsupply = 52.7o; Treturn,wet = 50.8o; Tsupply, wet = 59.4o). The measured sensible cooling capacity of the unit at a 77.4 degree outdoor temperature was 26,680 Btu/hr; the latent cooling capacity was 8,560 Btu/hr for a total capacity of about 35,240 Btu/hr. With a 4,181 Watt power draw; this works out to an EER of 8.4 Btu/W. This is short the nominal SEER of 10 Btu/W, but the indoor temperature was much lower (69.2oF) than the 80 assumed in the ARI test procedure.
The test at the PVRES home was done with the variable speed air handler (VSAH) operating at full speed. The unit was configured with the VSAH operating such that much of the time it operates at less than half speed. (For each cooling cycle, the unit operates at 50% flow for the first minute, then 80% flow for the next 7.5 minutes and finally 100% after that if necessary). Our experience so far shows that the unit seldom operates at full speed since measured on cycles last about six minutes.
Still we wanted to evaluate the unit operating at maximum speed where sensible heat gain would be at its highest and latent heat removal at its worst (a notorious problem in Central Florida). Readers may recall that we designed the duct system to have low resistance to flow. At full speed, we measured an air flow of 1380 cfm. The unit was operated in this mode for 20 minutes before measurements were taken; the outdoor inlet air temperature was 87(F. A 12.9 degree temperature drop was measured across the coil (Treturn = 70( [62( W.B.], Tsupply =57.1( [56D W.B.]) with a sensible cooling rate of 18,530 Btu/hr. Even at the high flow rate, latent performance was quite good with 8,770 Btu/hr of moisture removed.
Total capacity was 27,300 Btu/hr with measured power at 2074 Watts-- an overall EER of 13.2 Btu/W. The nominal SEER of the specific unit is 14.5 Btu/W. The rated capacity of the unit at the closest rated condition (85(F outdoor dry bulb, 72(F entering dry bulb, 63(F wet bulb) was 25,200 Btu at 900 cfm with an EER of 14.1 Btu/W. Given the higher outdoor temperature, the unit was very close to its rated efficiency.
For more information about the Florida Solar Energy Center's photovotaic residence in Lakeland, please visit them on the web at www.fsec.ucf.edu
The above article appeared in Today's Air Conditioning News, June 1999, courtesy of FSEC. Published by Today's Trade Publications, Inc. Today's A/C News serves over 16,000 readers in Florida and beyond. For more information contact Nick Willocks, publisher, 112 W. Pine Ave, Longwood, FL 32750. Ph: 407-332-4959, Fax: 407-332-5319, E-mail firstname.lastname@example.org.