RICHARD STOCKTON GHP SYSTEM: A CASE STUDY OF ENERGY AND FINANCIAL SAVINGS

L. Stiles, K. Harrison, H. Taylor

Richard Stockton College

Pomona, NJ 08240 USA

Tel: 1-609-748-5515

M. Sweikart

National Renewable Energy Laboratory

Boulder, CO 00001 USA

1. BACKGROUND

What is probably the largest single closed loop ground-coupled water loop heat pump system has been installed and been operational since January 1994 at Richard Stockton College. The energy savings, heat flow into and out of the closed loop well field, well field temperature and aquifer flow, as well as the microbiology of the aquifers and geochemistry are all being monitored. Computer simulation models for well field design are being tested for validity. The work reported here is on the energy savings and subsequent financial benefits while the remaining scientific results are reported in two other papers at this conference.

The well field consisting of 400 boreholes to a depth of 130m was completed in late August 1993, and the sixty-six rooftop (multizone DX) units were dramatically replaced in a one and a half day helicopter lift in late December 1993. The new units (GHP) with a total cooling capacity of 1400 tons (5300 kWc) were brought on line by January 18, 1994. The total space retrofit was approximately 35,000 m2 of a total 50,000 m2 in all the academic buildings. Since then another building has been added and tied into the well field adding an additional 180 tons to the system. The savings achieved for that building are not included in this analysis.

2. OVERVIEW OF MONITORING AND STATISTICAL ANALYSIS

Measurement of electrical energy of 16 HVAC zones representing over 50% of the capacity and on a five minute interval has been continuous throughout the period commencing in July 1993. Detailed baseline measurement of the gas portion of the heating load similar to the electrical load was not possible, only aggregated monthly submeter readings are available. Therefore heating loads can only be compared on a monthly basis, with only average outdoor temperature as an independent demand variable.

3. METHODOLOGY OF ESTIMATING SAVINGS

Electrical Savings are estimated based on a detailed statistical analysis of the use of the rooftop units before replacement from August 1993 through December 15, 1993. The data was separated into cooling season (through October 15th) and heating season (after October 15). The daily use of each of sixteen zones (wing & floor) were regressed on the daily average inside and outside dry bulb and wet bulb temperatures, wind speed, solar insulation, time of operation, and scheduled occupancy. These sixteen individual regressions were performed separately for cooling and heating seasons pre-replacement and served as the baseline for post-replacement analysis. The electrical daily peak power demand was likewise determined for the cooling season based on the same regression variables and served to determine the reduced power demand of the replacement project. The metered space represents 50% of the area served by the total replacement project and 52% of the installed capacity. So the measured savings of the sixteen zones are normalized by .51 to project the estimated savings for the entire project. To estimate the electrical savings for the individual zones, a model estimate of the original HVAC system use based on the above statistical analysis is compared with the actual use of the replacement system on a daily basis. If there was any data out of physical range during a single day the entire days worth of data was rejected. The reduced data set was assumed to be representative of the entire month, so the daily average was used to estimate a month’s savings. To properly credit the use of the pumping energy, the main pump flow was monitored. From this data the fraction of pumping energy was estimated and applied to the entire project.

Baseline gas usage was determined by regressing the monthly gas use of the academic buildings on average outside temperature for three years prior to the replacement project. This became the model for predicting what the pre replacement unit would have used under similar operating conditions. Since the library project in 1995 (an addition of 4,000 m2) was completed, the gas use estimate was increased based on a ratio of the new floor area to the old.

The replacement system has several features that result in reducing energy use. Besides the increase in efficiency for heating and cooling, there is an added savings from the reduction of operating hours achieved through a better energy management system. In addition, the inside temperature setting can be raised in the cooling season and lowered in the heating season due to increased comfort from the replacement system without sacrificing comfort to the occupants. These latter factors become evident in the second twelve months after the energy management system became fully operational. These savings are analyzed separately and added to the savings found from the increased efficiency alone.

There are several features responsible for energy savings besides the improved efficiency of the heat pumps over the original equipment. A major aspect of the new system is variable speed high efficiency water pumps. A primary water loop to the field is often throttled to a very low value especially when there is a mixed heating and cooling demand. Approximately 25 % of the savings could be lost if there were no controls on the pump use as would happen if the pumps always ran “flat out”.

4. DISCUSSION OF RESULTS (Note that this section could be incorporated in the above section as appropriate)

A detailed analysis was performed on the system as operated in the second twelve month period (April 95 - March 96) compared with the previous twelve month period. It was assumed that the first period represented a control similar to the original system, while during the second period, the computer energy management system (EMS) was controlling the system more efficiently. Indeed this was found to be the case as is shown in Figure 1. It was found that the HVAC system was operated fewer hours and often the temperature in the buildings was either higher in the summer and lower in the winter compared with the previous year. These are shown as time and temperature savings. The remaining savings are those due to the operational efficiency of the GHP system. It should be noted that the temperature savings are due to the increased comfort provided by the GHP system compared with the original. Without the time reduction there would have been more efficiency savings because the original system would demand more energy, so it isn’t quite correct to assess the time savings entirely to the EMS.

The electrical energy savings are essentially those projected by the feasibility study, while the gas savings fall short by about 25%. This is due to the continued use of gas during the unoccupied periods in the night when the GHP system is shut down. A better use of the GHP system and less of the gas - fired perimeter heating system during these periods would result in the projected savings and better balance the thermal load on the well field. Figure 2 shows the gas savings from October 1994 to March, 1996.

From May, 1994 to March, 1996 the total electric savings was 3,203 MWh. The dollar value of this savings at a marginal cost of 0.0808 was $258,400. Additional savings from peak load shaving (kW) was $107,100 for a grand total of $365,200. Thirty-eight percent of the savings came in the first year of operation of the project while sixty-two percent came in the second, when the project was fully operational. Broken down by heating (October through March) and cooling (April through September) seasons, the electric savings were twelve percent and eighty-eight percent respectively.

Gas savings (T) during this same period totaled 180,700 therms, which were valued at $119,440. The lion’s share (sixty-one percent) of the gas savings came in the second year partially due to technical (the gas perimeter heat was not turned off during the first heating season) and climatic (a substantially colder winter during the second year of operation) reasons.

Figure 1: Monthly Electric Savings


Total dollar savings (cost avoidance) through March 1996 are $488,000 with sixty-three percent of the savings coming in the second year of operation of the project. An analysis of the second year alone shows a total dollar savings of $307,000 of which eighty-seven percent came from electric savings and thirteen came from gas. In terms of seasonal savings, sixty-eight percent of the savings came in the cooling season, while thirty-two percent accrued during the heating season.

Figure 2: Monthly Gas Savings

Assuming an annual dollar savings of $300,000, the present value of the cost avoidance with a twenty-five year probable usefulness life and a real rate of interest of four percent is equal to $4.7 million dollars. If the project has a thirty year life savings total $9 million and the present value is $5.2 million. This analysis does not include an estimate of expected reduced cost in maintenance and equipment replacement which was not measured but could well increase savings an additional twenty-five percent or more on an annual basis. Furthermore, the analysis assumes fuel costs will remain constant in real terms.

Empirical Findings 1992-1997


The 1992 engineering estimate projected a total annual gas and electric savings of US $312,000. Our analysis of the second year of operation of the system using a detailed statistical model showed annualized savings of $307,000. This section looks at the actual data for gas and electric usage from 1992 through 1997 and calculates the total and annual savings for this period. We then compare these actual metered results with the model results reported above.

Figure 3: Gas Usage from 1992 to 1997.

Figure three represents the total gas usage from 1992-1997. Total CCF was 283,170 for 1993, peaked at 309,222 for 1993 and then steadily declined to 210,238 in 1997, a decline of 32% since the system was fully operational in 1993. It is important to recognize that this decline took place while the square footage of the project increased about ten percent by virtue of having the library addition come on line in 1993 and increased utilization of the project area. The overall gas savings from 1993, valued at 51 cents per therm, are estimated to be $147,800 without taking the increase in building size into account. This number should be adjusted upward to $209,700 when the 10% increase in floor area due to a library addition is considered.

Figure four represents the total electric usage from 1992-1997. Total kWh was 7.37 million for 1992, rising to a high of 8.12 million kWh in 1994 and then declining to 7.22 million kWh in 1997, a decline from peak of eleven percent. The reduced electric energy demand from 1994 is estimated to be, valued at nine cents per kWh, $179,000 without taking the increased building size into account. This number should be adjusted upward to $471,100 when the library addition is considered.


Figure 4: Total Electric Usage from 1992 to 1997. Dotted line represents estimated usage based on increased floor area in 1994 due to addition to Library.

Figure 5: Peak Demand from 1992 to 1997. Dotted Line represents increase due to Library addition and new Arts and Sciences building.

Figure 5 represents the annual peak load demand for 1992-1997. Total kW was 43,248 for 1992, rising to 45,780 for 1993 and then steadily declining to 37,716 kW for 1997, a decline of thirteen percent from 1992 and eighteen percent from 1993. The dotted line drawn across figure 5 represents the estimated increase of peak demand

due to the addition of the Arts and Sciences building and the Library addition (1994) of about 3800 kW and the area under the straight line is the amount of peak savings which total 26,976 kW. Valued at $9.50 per kW saved, this savings amounts to about $256,300. When the library addition and new Arts and Sciences building is considered this results in a further savings to $333,000.

For the latest year, 1997, the total savings are estimated to be $207,700, when directly compared with the use in 1993. When the library addition is factored into the calculations, the total cost avoidance is estimated to be $317,900. This does not include reduced maintenance costs and the economic value of less pollution generated. The total savings to date, according to these calculations, amount to $583,100compared with the 1993 use and $1,018,000 when adjusted for the library addition. This analysis is shown in Table 1.

Table 1: Total Savings Based on Projected Increase from 1993 (in $1,000)

Year / Gas / Electric / Total
1994 / 43.6 / 139.5 / 183.1
1995 / 48.4 / 182.8 / 231.2
1996 / 51.9 / 230.0 / 281.9
1997 / 65.8 / 252.1 / 317.9
TOTAL / 209.7 / 804.4 / 1014.1

5. FUTURE SAVINGS

Future savings will depend on the consensus forecasts of world energy prices and the proposed restructuring of the electric and gas industries in the United States. The most recent estimates from the Energy Information Administration /Annual Energy Outlook 1996 expect real gas prices to rise (1990 dollars) 6% per year from 2000 to 2010 (low estimate) or 20% per year during that same time period. (high estimate) The high number assumes a real economic growth rate of 2.7% per year and that imported oil prices will increase to $33 per barrel (in 1990 dollars). The low number for the year 2000 assumes the same oil price but a lower real economic growth rate of 1.8% per year. The low number for the year 2010 assumes that the price of oil will be $23 per barrel in 1990 dollars and by 2010 the annual growth rate will be above 2.2%.

Electricity prices (in 1990 dollars) are expected to rise about 11% per year (low estimate) to around 6.8 cents per kWh while the high estimate is expected to grow at the same rate (11%) but cost between 7.1-7.2 per kWh by 2010. The high number assumes prices for oil will rise to $33 per barrel by 2010 (1990 dollars) and that real economic growth will be 2.7% per year. The low number assumes that the oil price is the same as the high estimate but real economic growth is assumed to be 1.8% per year.

6. CONCLUSION

Our findings are consistent with the original 1992 engineering estimate which projected an total annual savings of US $312,000 compared with the original system. By comparing the results in 1996, it appears that the actual savings which is estimated from the monthly meter data is lower than that from the statistical modelling by about 20%. We believe this is probably due to the college taking some of the savings in increased comfort. That is, the statistical model measures the actual operational effects related to comfort while the monthly electrical and gas metered data does not.

While we cannot compare the projected savings of the GHP system with a standard replacement project, we believe the results here give confidence that those figures are also correct. In particular the projections were that the incremental savings would pay back the incremental capital costs in five years.

ACKNOWLEDGMENTS

We wish to express our deep appreciation to Atlantic Electric (AE), the Electric Power Research Institute (EPRI), the South Jersey Transportation Authority, and Sandia National Laboratory (SNL) for funding this project. We are especially indebted to Lew Pratch (US DOE) and Dr. William Sullivan (SNL), Dr. Hwangwei Siang (AE) and Dr. Mukesh Khattar (EPRI) for their help in the initial stages of funding. Our special thanks go to Dr. Charles Tantillo and his staff, Robert Hannum and his staff, Pibero Djawotho, David Bryan, Gregory Stevens, Bret Becker, and Linda Stafford for all their help.

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