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Geothermal & You - Economic Benefits

Case Study

Detailed case study: Alliant Energy's operating facility in Galena, Illinois

Summary of Economic Benefits
Abstract
Introduction
Site Description
Purpose & Expected Benefits
GCHP System Solutions
Measured Performance Results
Economic Analysis
Conclusions & Implications

"I used to need an electric space heater under my desk, but now my feet stay warm. The guys can work in the shop area in shirt sleeves all year long and the system is clean with no dust."

- Barb Schaible, Office Manager

Summary of Economic Benefits

  • Payback Period: 8.9 years
  • Return on Investment: 11%
  • Cost of Project: $10,570
  • Annual Energy Savings: 23,570 kWh
  • Annual Energy Cost Savings: $1,193

Abstract

Three ground coupled heat pump (GCHP) units were installed at the Alliant Energy office in Galena, Illinois in the fall of 1994. These units were installed to save energy and operating costs over the alternative electric heating system.

The Alliant Energy Galena office consists of an office area that is both heated and cooled and a shop area that is only heated. Both areas are heated by hydronic heating in the floor. Meters were used to measure the electric energy consumption of major system components.

The results of the metering showed an average annual cooling energy consumption for the GCHP system of about 1.09 kWh/ft2 and an average annual heating energy consumption of 5.17 kWh/ft2. This annual energy use results in a cost for cooling of about $0.064/ft2 and for heating of about $0.26/ft2 for a total of $0.324/ft2 .

The annual energy cost of alternative electric heating and conventional cooling systems for the same facility is estimated to be $0.703/ft2 . The incremental cost of the GCHP system installation is estimated at $10,570. The estimated annual energy cost savings for the GCHP system is $1,193. Therefore, the payback for the GCHP system is 8.9 years.

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Introduction

This is a case study of a ground coupled heat pump (GCHP) system installed in the Alliant Energy office and shop in Galena, Illinois. The general advantage of a GCHP system is the use of the ground as both the heating and cooling source for the building.

The ground temperature is around 50 degrees F all year which makes it more efficient than the outdoor air temperature to use as a heat source in the winter and as a cooling source in the summer.

Electric meters were used to measure energy consumption of the GCHP system for three years. The data collected is used in this report to demonstrate the benefits of the GCHP system compared to conventional electric heating and cooling systems.

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Site Description

The building consists primarily of a shop area and an office area. The shop and office areas are heated by two GCHP hydronic systems that use pipes in the floor to distribute the warmed system fluid to heat the space. The hydronic system does not provide cooling and there is no cooling in the shop area.

The office area is cooled by a forced-air GCHP unit. The forced-air unit in the office area is used to provide back-up heat if the hydronic heating is not enough to maintain comfort in the office.

The heating of hot water for the facility is supplemented by a desuperheater on the office forced-air GCHP unit coupled to an electric resistance water heater. Pertinent site information is summarized in Table 1.

Site Information - Table 1

Building Type:

Small Office and Shop

Equipment Configuration:

Two 4-ton GCHP units serving two Wirsbo hydronic heating systems 3-ton GCHP unit with desuperheater  serving forced-air system

Building Configuration:
Office Area:
Shop Area:


1,231 ft²
2,369 ft²

Average Electric Rate:

$0.059/kWh summer;
$0.050/kWh winter

Date Meters Installed:

9/23/94

The site is located in northwestern Illinois which has typical heating degree days (HDD) of about 7400 (base 65oF) and a typical cooling degree days (CDD) of 700.

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Purpose and Expected Benefits of GCHP System

The purpose for the installation of the GCHP system was primarily to use the ground thermal heat to reduce the electric heating costs for the building. It was decided to install the GCHP units because the energy efficiency of the system was three to four times the efficiency of the electric heating system alternative.

The system also was expected to save cooling energy for the office area with a higher EER than a conventional air conditioning system, but this savings was expected to be much smaller than the savings for the heating system for both the office and the shop area.

The desuperheater on the office unit is installed to use waste heat for the hot water system. When the unit is used for cooling the office area, some of the heat is rejected into the water heating system reducing the amount of heating required by the electric water heater.

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GCHP System Solution

The GCHP system installed uses three units with a total cooling capacity of 3 tons and a total heating capacity of about 120,000 btu/hr. This system uses vertical ground coupling with 14 ground loop wells to a depth of 65 feet. This ground coupling system supplies heating energy to all three heat pump units.

The general operation of the system is to use the two four-ton water-to-water heat pump units to heat the water for the hydronic floor heat system in the shop area. The three-ton heat pump unit was used to heat and cool the office space with energy obtained from the ground loop system. In the cooling mode, the office unit rejects some heat to the water heater before the rest of the heat is rejected to the ground loop.

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Measured Performance Results

On September 23, 1994, meters were installed on each of the three GCHP units. Meters were also installed on the two hydronic floor pumps, the ground loop pump (which serves all three GCHP units), the desuperheater pump, and the resistance water heater. Each meter recorded kWh consumption. Electric demand was metered for the two hydronic GCHP units, the two floor pumps, and the ground loop pump.

The electric energy consumption of the system was measured over the three year period from 1994 to 1997. The energy required for cooling and heating was divided based on observed heating and cooling degree days during the year.

The top half of Table 2 shows the results of the measured energy consumption for cooling in the office area for the three years. The bottom half of Table 2 shows the results for each year when normalized to the thirty year average of 702 cooling degree days (CDD).

Cooling Summary - Table 2

 

 

Observed Cooling Energy

Cooling
Season

Observed
CDD

kWh

Cost

kWh/ft²

Cost/ft²

1995

1996

1997

986

689

679

1,591

1,518

1,285

$94

$90

$76

1.29

1.23

1.04

7.7˘

7.3˘

6.2˘

Cooling Season

30-year Average CDD (Waterloo)

Weather-Adjusted Cooling Energy

kWh

Cost

kWh/ft²

Cost/ft²

1995

1996

1997

702

702

702

1,132

1,547

1,328

$67

$92

$79

0.92

1.26

1.08

5.5˘

7.5˘

6.4˘

The average weather-adjusted annual cooling energy for the three years is 1,342 kWh which results in an energy intensity of 1.09 kWh/ft². This is less than half the average for all commercial offices in Alliant Energy's service territory of 2.34 kWh/ft².

The top half of Table 3 shows the results of the measured energy consumption for heating in the office and shop areas for the three years. The bottom half of Table 2 shows the results for each year when normalized to 7,406 average heating degree days (HDD).

Heating Summary - Table 3

 

 

Observed Heating Energy

Heating
Season

Observed
HDD

kWh

Cost

kWh/ft²

Cost/ft²

1994-95

1995-96

1996-97

6,696

7,783

7,693

16,683

20,370

18,703

$832

$1,016

$933

4.63

5.66

5.20

23.1˘

28.2˘

25.9˘

Heating Season

30-year Average CDD (Waterloo)

Weather-Adjusted Heating Energy

kWh

Cost

kWh/ft²

Cost/ft²

1994-95

1995-96

1996-97

7,406

7,406

7,406

18,451

19,372

18,011

$920

$966

$898

5.13

5.38

5.00

25.6˘

26.8˘

24.9˘

The average weather-adjusted annual heating energy for the three years is 18,612 kWh which results in an energy intensity of 5.17 kWh/ft². This is less than half the average of 11.29 kWh/ft² for all commercial offices in Alliant Energy's service territory with electric heat.

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Economic Analysis of GCHP System

The measured energy consumption of the GCHP units allows an accurate estimate of the true energy savings for the system.

The results of the metering showed an average annual cooling energy consumption for the GCHP system of about 1.09 kWh/ft2 and an average annual heating energy consumption of 5.17 kWh/ft2.

This annual energy use results in a cost for cooling with the GCHP system of about $0.064/ft2 compared with a typical conventional cooling system cost of $0.138/ft2. Therefore, the annual cooling energy cost savings for the GCHP over the conventional system is $91.

The annual energy cost for heating with the GCHP is about $0.26/ft2 which compares to the cost with a typical electric heating system of $0.565/ft2. This difference in cost results in an annual heating energy cost savings of $1,102. Therefore, the total energy cost savings for the GCHP system is $1,193.

The cost savings for the desuperheater was too difficult to accurately estimate. A rough estimate of the savings would be about $25 per year. This rough savings estimate is not used in the calculation of the payback for the system.

The cost to install the GCHP system was about $14,980. This cost includes the two heat pumps for the shop area, the pumps for the loop system, and the ground loop wells. The cost of the alternative electric heating system was about $4,410.

Therefore, the incremental cost of the GCHP installation was $10,570. With the annual savings of $1,193, this results in a payback for the GCHP system of 8.9 years and a return-on-investment of 11%.

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Conclusions and Implications:

This case study shows that for this application of GCHP the payback is about nine years. The payback would be reduced to under 8 years if the site required cooling for the shop area.

The heating energy savings is typically much larger than the cooling energy savings, but for this site there is a greater difference because the shop area is only heated.

Rebates are available for Alliant Energy customers in Iowa and Illinois for GCHP systems. Because this is an Alliant Energy building no rebates were available. If this had been a customer site the rebate would have been $3,825, which would have reduced the payback to 5.7 years.

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