Operating Experiences of Geothermal Heat Pump Convenience Store VS Conventional Roof Top HVAC Convenience Store

by Steven C. Ganzer, Geothermal Design & Engineering

ABSTRACT

There are many geothermal heat pump installations in commercial and institutional buildings in the United States and Canada.  But little information is available about their merits, design details, ground heat exchanger design and sizing, integration with traditional building system, and performance.  The information being presented includes description of the buildings, HVAC systems, refrigeration equipment, project costs, system's performance, operating difficulties, maintenance costs, and owner satisfaction.  The information was obtained through direct contact with each convenience store by the way of a building automation system, and owner's personnel.

INTRODUCTION

Geothermal ground loop heat pump system is one of the cleanest, safest, and most cost efficient heating and cooling alternatives available.  The geothermal system is a simple but elegant design consisting of one or more water source heat pumps and an earth heat exchanger (commonly called a ground loop).  The earth heat exchanger is designed to utilize the earth with its relatively constant temperature, as a heat source and/or a heat sink.  Geothermal assisted water source heat pump technology takes full advantage of this self sustaining thermal storage capacity of the earth.  Depending upon weather you desire heating or cooling, heat is either extracted into the near constant temperature of the earth as opposed to an air source system that rejects at into or gathers heat from the temperature extremes of the outside air.  The water is recirculated through polyethylene pipe absorbing heat from the warmer earth during the winter, and transferring heat to the cooler earth in the summer.  Since the system moves existing heat energy, heating and cooling functions are more efficient and environmentally sound.

When the building demand calls for heating, water pumped through the loop absorbs heat from the relatively warm earth.  The heat is moved via the loop into the building where water source heat pumps direct warm air to those areas calling for heat.

To cool, the heat pump (a compressorized refrigeration system) removes heat from the building and transfers it into the temperate earth via the loop.  Because of the relative cool temperature of the earth, heat is readily and efficiently absorbed from the loop.

This means water source heat pumps in a geothermal system can offer substantial operating savings over other types of air-conditioning systems which must reject heat into outside air.

Although geothermal heat pumps are an effective means of reducing building energy consumption and electric loads peaks, mechanical design teams at large need more information about them before they will be confident enough to consider geothermal heat pump systems for clients.

This paper is intended to provide information on the use of geothermal for commercial sites.

SITE SELECTION

Texaco's construction manager, Jerry Buri, for the Oklahoma region was approached about using geothermal heat pump system technology to heat/cool one of his convenience stores.   The decision was that Texaco would heat and cool one of their co-branded (chain restaurant inside the store) stores using geothermal heat pump technology.  The store's electric usage will be measured and compared to a similar store using conventional roof top HVAC equipment to heat and cool.

The two stores are located in Oklahoma City approximately five (5) miles apart.   The store using geothermal has the entrance oriented West and the conventional store's entrance is oriented East.  The conventional store has natural gas heat and natural gas cooking in the restaurant while the geothermal store is all electric including the restaurant cooking.

GEOTHERMAL HEAT PUMP INSTALLATION

Geothermal Heat Exchanger Design

The design of the earth heat exchanger depends on many factors, including local climate, ground composition, moisture content and temperature, ground conductivity, and the building's air conditioning, and building loads, refrigeration equipment load, hot water usage.  Typically it is a series of high density polyethylene pipe buried in one of several loop configurations below the surface of the earth or in a body of water through which water (sometimes mixed with antifreeze) is circulated.

The ground heat exchanger is site specific designed and the Texaco site the following design parameters were used:

1. Thermal conductivity of 1.4;
2. Earth's constant temperature of 62°;
3. Volumetric heat capacity of the ground 35.0;
4. Volumetric heat capacity of the fluid 62.4;
5. Borehole thermal resistance .173;
6. Lot size of 280' x 280';
7. Existing car wash to remain;
8. Building load of 12 tons;
9. Refrigeration load of 165,394 peak BTU's per hour;
10. Horizontal ground heat exchanger to vent 140,560,000 BTU's annually (refer to table 1);
11. 60,000 BTU's per hour;
12. 24 hour store operations;
13. Restaurant exhaust requirements;
14. Total gpm flow of 80 gpm; and
15. Car wash to vent 231,540,000 BTU's annually (refer to table 2)

    • 80° to 90° hot water;
    • 1,000 gals. per month;
    • 35 gals. per basic car wash;
    • 59.4 gals per deluxe car wash; and
    • 76 gals. per works car wash.

Table 1. Design Parameters Of Using The Horizontal Ground Heat Exchanger As a Vent

Table 2. Design Parameters Of Using Car Wash Hot Water As a Vent

The ground heat exchanger design for the convenience store is 12,000 lineal feet of 1" polyethylene pipe in a "U" bend configuration.  The layout configuration that we used was 20 - 3 5/8" boreholes of 300' spaced on 30' centers.   The ground heat exchanger is designed for fifteen (15) years or 180 months which is the normal time of operation before raise and rebuild is constructed on the facility.   The maximum peak temperature of the supply water temperature is 105.6° in month 177 and a minimum peak water supply temperature is 61.91° in month 2.

The horizontal ground heat exchanger configuration was five (5) slinkys placed flat horizontally just under the concrete slab located in front of the store's entrance, the entrance and exit of the car wash, and one (1) under the concrete sidewalk in front of the store.

Building Design

The geothermal convenience store has 3,528 square feet which is made up of:

1. 2,410 square feet of sales area;
2. 250 square feet of office space;
3. 400 square feet of storage;
4. 192 square feet of walk-in cooler;
5. 64 square feet of open face dairy case;
6. 64 square feet of restaurant freezer;
7. 64 square feet of restaurant cooler;
8. 1,000 square feet of car wash;
9. The building exterior wall height of 14 feet;
10. 6" walls with 6" insulation;
11. exterior stud wall had a 8" concrete block veneer; and
12. 8' and 11' ceiling heights consisting of free formed lying ceiling tiles.

 

The refrigeration equipment consisted of water cooled compressors of the following sizes:

1. Walk-in cooler 5hp, 73,500 peak BTUs per hour;
2. Dairy case 2hp, 10,400 peak BTUs per hour;
3. Restaurant freezer 3/4hp, 9,194 peak BTUs per hour;
4. Restaurant walk-in cooler 2hp, 10,400 BTUs per hour; and
5. 2 Fountain machines 13,200 peak BTUs each hour (26,400 BTUs total)

Table 3. Designed Refrigeration Equipment's BTU Monthly Load Profile

Month	BTU's
January	73,663,000
February	36,880,000
March	50,438,000
April	62,463,000
May	72,500,000
June	80,512,000
July	85,987,000
August	88,850,000
September	89,013,000
October	86,471,000
November	81,302,000
December	73,663,000
Total	881,742,000

The geothermal store is designed with 3-51,250 total BTU horizontal heat pump equipment with an ARI 330 rating at 70° entering water temperature of 15.0 energy efficient rating and a coefficient of performance of 4.4.  The water source heat pumps are installed between the ceiling and roof joints allowing easy access for routine maintenance.   Outside air requirements of ASHRAE 62-89 are met by the use of a common outside air duct with individual duct taps for each heat pump.  The equipment physical dimensions allowed the exterior walls and ceiling design to remain as originally designed.  The roof structural members were decreased in size as well as in number required.

Table 4. The Building Peak Block Monthly Design Loads

The interior piping system consists of copper piping ranging from 2" to 3/4" in size.  The field supply line enters the building; connecting to supply side of the water source heat pumps and the refrigeration equipment.  The return water from the refrigeration equipment and the convenience store water source heat pumps is pumped through a shell and tube heat exchanger to heat the domestic hot water stored in a 100 gallon storage tank to be used in the car wash.  The return water after passing through the heat exchanger flows back to the field (ground heat exchanger).

The supply and return water piping for the water source heat pump in the car wash is for heat only.  The horizontal piping heat exchanger is 3/4" polyethylene slinkys without any antifreeze.  There are five (5) of these located as follows:   1 at the store's entrance; 1 between the store and fuel pumps; 1 at car wash entrance; and 1 at car wash exit.  There is a three way value that when the outside air temperature reaches 25° it opens allowing the return water to flow through the horizontal heat exchanger at a flow rate between 20 gpm and 30 gpm and giving approximately a 30° delta T.

Water Source VS Air Source

The water source heat pumps that we selected for this building have a spine fan coil and the air source roof top equipment at the conventional store that we are comparing the geothermal store to has a plate fin coil.  According to a California Energy Commission independent study after eighteen (18) months both coils have suffered significant loss in capacity when not cleaned to manufacturers recommended maintenance procedures over a period of time.

The spine finned coil loss for our water source heat pumps is estimated at a 10% efficiency over eighteen (18) months of no maintenance and after cleaning we will recover 70% of the lost for a permanent net loss of 3% efficiency.

The plate fin coil loss of the air source roof top equipment at our conventional Texaco convenience store is estimated at a 27% efficiency over eighteen (18) months of no maintenance and after cleaning will recover 67% of the loss for a permanent loss of 9% efficiency.

Table 5. Geothermal (All Electric) Convenience Store Energy Consumption

Table 6. Conventional Roof Top (Duel Fuel) Convenience Store Energy Consumption

Table 7. Convenience Store HVAC and Refrigeration Installation Building Cost Comparisons

Table 8. Convenience Store Operating Cost Comparisons

NOTE:

1) Difference in HVAC Installed Cost divided by the Difference in Operating Cost = Simple Payback or $46,000 divided by $11,430.09 = 4 Years

2) Difference in Total Costs divided by the difference in Operating Cost = Simple Payback of all Associated Costs or $52,000 divided by $11,430.09 = 4.5 Years

Table 9. Geothermal Convenience Store VS Air Source Conventional Convenience Store Performance Savings

* The quality restaurant was closed this month in the conventional store located at 501 N. Meridan.

CONCLUSION

Performance

The supply water from the earth heat exchanger is entering the building at a temperature ranging 58º to 91° which is standard range heat pump equipment with a Delta T of 10° to the refrigeration and water source heat pump equipment.  The refrigeration equipment and return water from the water source heat pumps is providing 87° water from the shell and tube heat exchanger to the 100 gallon storage tank for the car wash.  The hot water is supplied to the car wash at 30 gpm at a 50psi.  On the average day there are 102 cars washed using a total of 6,233.6 gallons.  These gallons convert to 44,790,00 BTUs per month for a yearly total of 537,480,000 BTUs.

1) Number and type of car washes on the average day

a) 80° to 90° hot water.                                                                                               b) 24 deluxe car washes at 35 gals. per basic car wash;                                               c) 29 super car washes at 59.4 gals per super car wash; and                                       d) 49 works car washes at 76 gals. per works car wash.

Table 10. Car Wash's Hot Water As A Vent Performance For One Year

The run times of the water source heat pumps range 75% to 85%.  The run times of the refrigeration equipment varies from 30% to 60%.

Economics

The HVAC installed cost difference between geothermal closed loop heat pump convenience store and the conventional air source roof top convenience store is $46,000 (ground heat exchanger $40,000 plus $6,000 for interior piping).

The Simple Payback of the geothermal closed loop heat pump system at Texaco is 3.75 years.

1) Difference in HVAC installed Cost divided by the difference in Operating Cost = Simple Payback or

                ($82,000-$36,000) divided by $12,249.84 = Payback

                                or $46,000 divided by $12,249.84 = 3.75 years

Life-Cycle cost analysis for the payback of the geothermal system for Texaco gives you 3.42 years.

1) Difference in HVAC Installed Cost - Difference in Equipment Cost divided by Difference in Operating Costs + Difference in Maintenance Costs = Life Cycle Payback

                        ($46,000 - ($4,000 - $1,200) divided by ($12,249.84 + $40.00)

                                   $42,000 divided by $12,289.84 = 3.52 Years

The geothermal convenience store will realize additional savings of approximately $161,164.80.

1) Reduced construction costs of $8,700:

2) Investing the $12,249.84 annual energy savings at an interest rate of 15% for eleven (11) years will earn approximately $140,873.66;

3) Investing the geothermal HVAC equipment cost of $6,000 at 15% for five (5) years will earn $12,068.14.

LESSONS LEARNED

Through the use of project management we found the following lessons to be true:

Coordination to be the most important issue to meet the owners construction time.   The normal construction  time of 90 days was met by coordinating the driller's access to the site while the general contractor is able to build the geothermal convenience store in usual manor (installing site utility lines, constructing the building pad, etc.) of building the conventional convenience store.

If the Texaco conventional convenience store does not implement a program of through coil cleaning on a regular basis, its two (2) 10 ton air source roof top units with a 10 energy efficient rating (EER) will quickly become a 7.5 energy efficient rating.

For a quarter of a century, air source split and packaged HVAC equipment has been the industry norm.  The practice of applying a "Factor of Safety" to building HVAC designs compensates for this decay in performance.  The short fall in cooling capacity may prove to be significant at higher than design ambient temperatures.   These higher ambient temperatures often leads to short cycling and general loss of dehumidification of the facility.  In the case of our convenience store it leads to water running down the inside of our cooler doors.

Compared to alternative methods of heat rejection, the average air source system is over designed.  WE found it was possible to do more with less.

Construction Costs

Additional Construction Costs

The construction of the geothermal convenience store raised the normal construction costs by $49,700

1) Ground heat exchanger material, drilling and installation of the ground heat exchanger at a cost of $36,000;

2) Material and installation of horizontal ground heat exchanger (6 slinkys) at a cost of $4,000;

3) HVAC interior piping at a cost of $6,000;

4) Water source heat pump equipment cost an additional $100 per ton more than air source (12 tons @ $100) at a cost of $1,200; and

5) Material, building, and installation of metal refrigeration rack for cooler and freezer water cooled compressors at a cost of $2,500.

Reduced Construction Costs

The construction of the geothermal convenience store reduced normal construction costs by $9,200 in the following ways:

1) Reduced the number of roof joints and labor (6 @ cost of $500 each) by a cost of $3,000;                                                                                                            

2) Reduced the cost by using lighter weight roof joints (42 @ $50 each) for a cost of $2,100;

3) Increase in refrigeration efficiencies allowing a reduction in the size of compressors a savings of $2,000;                                                                                                                                          4) Reduction in roof penetrations for natural gas usage and air source roof top units, a savings of $2,100;                                                                                                                                         5) Reduction in plumbing by not having to run any natural gas piping air source roof top equipment and restaurant cooking equipment, a savings of $2,000; and  

6) The reduction in HVAC equipment tonnage from 20 tons to 12 tons (8 tons @ 500 per ton), a savings of $4,000.

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