A Ground Source Heat Pump Solution for Colorado State University

Background

Colorado State University (CSU) installed a ground source heat pump (GSHP) system to provide heating and cooling to its 420,000-square-feet Moby Athletic Complex. The Moby Complex contains an 8,000-seat basketball arena, NCAA swimming pool, practice gyms, sports medicine labs, coaching offices, locker rooms and more. The new GSHP system replaced 1960s-era systems, two pipe hydronic system fed by steam and air-cooled chillers, with a ground source heat pump system, allowing the university to abandon approximately half mile of old steam and condensate piping that was only serving this building.

Following the completion of this project, the Moby Athletic Complex reduced total energy use by more than 50%. Because nearly all the energy use for the building is now provided by electricity, and the local electric grid is headed to 100% renewable electricity, the Moby Athletic Complex will be a nearly carbon-free building by 2030.

Solutions

Updating aging equipment and improving efficiency are foundational goals for the university, but CSU Facilities Management staff realized replacing the aging equipment at the Moby Athletic Complex with “business as usual” equipment would not be in the best long-term interest of the University.

The bore field, located below a section of adjacent intramural sports fields, consists of 342 wells, 550 feet deep, and contains over 70-miles of closed-loop piping. A GSHP provides efficient heat and cooling by taking advantage of the constant temperature of the earth. Circulating liquid through the bore field piping, the earth serves as a large heat source or sink. At the time of construction, this system was one of the largest ground source heat pump projects west of the Mississippi River.

When CSU FM staff began studying ways to update the aging HVAC system in the Moby Complex, several factors were considered:

Many of the HVAC components were original to the building (1966) and were past end of useful life.
All buildings except the Moby Athletic Complex had already been removed from the district steam system on the west side of the campus. It was a goal of this project to move away from steam and allow FM to abandon nearly a half mile of aging steam and condensate piping.
There was not adequate space in the existing mechanical rooms of the Moby Athletic Complex to house traditional boilers and chillers. If that “business as usual” option was chosen, a building addition would be required to house the equipment. And, because Moby serves several sports, there is no “back door” to the facility. Any addition would be an eyesore to one of the entrances.

The university conducted a feasibility study (including a test well) that demonstrated to facilities staff that the GSHP was the best life cycle cost though it did have a higher upfront cost. The feasibility and life cycle cost assessment were completed in 2013; however, funding was not secured until 2018. Though funding was deferred, it was secured in time to avoid expensive upgrades to the old steam pipes and major HVAC components.

When the project was approved, facilities staff secured a GSHP expert to be the owners’ representative throughout the installation process. They were an invaluable resource for staff who were not yet familiar with the multitude of details of a successful GSHP system. The design build team also had a GSHP expert. During weekly project team meetings both facilities staff and the design build team learned from these two experts.

Testing the piping in the bore field was completed at three different phases of the project to ensure there were no leaks. However, when the GSHP came online, the system was requiring substantial makeup water due to a leak. The leak stemmed from poor fusion on the 16” HDPE pipe that runs from the bore field into the building. The leaking elbow was repaired under warranty and the makeup water meter has gone to zero.

The heat pumps selected deliver 140 °F water for space heating. CSU’s standard requires DHW temperatures of 140 °F or higher, so the team decided early on to maintain natural gas water heaters in the building. This is a small portion (11%) of the overall building load and avoids operating the heat pumps at a higher temperature, which makes them less efficient.

When the building was served by steam, the steam to hot water converter at the building entrance delivered 180 °F water to the hydronic components within the building. During the design phase, FM maintenance staff worked with the designers to adjust the steam to hot water converter to drive that hydronic temperature down – and to test how well the HVAC equipment worked at lower temperatures. Fortunately, the old system was over-designed so no dramatic increases in coil sizing were required.

While investigating the advantages and disadvantages of a GSHP, facilities staff learned that the diverse loads within the Moby Athletic Complex were a good match for heat pump technology. For example, when there is a large crowd in the basketball arena and cooling is needed, excess heat can be sent across the building to other loads like the swimming pool. Frequently, heat is being moved around the building with no need to exchange heat with the bore field. On days when the heating or cooling load exceeds what can be used in other parts of the building, the heat is transferred to (or extracted from) the bore field. The design build team suggested that the sharing of heat within the building is not well represented by the energy models, and higher savings should be expected. CSU staff was pleased to learn the team was correct – the system is performing better than modeled.

There are different ways to design a GSHP system – some utilize an “ambient loop” where ~50 °F degree water from the bore field is circulated throughout the building and heat pumps are located throughout the building at the point of use. The other method, which is the one Colorado State University chose, is to bring the ambient loop to a mechanical room in the building where all the heat pumps are located, and then hot and cold water from the heat pumps are distributed with a four-pipe hydronic system throughout the building. The primary reason for the centralized heat pump approach was to have the equipment accessible for maintenance and to allow the team to utilize some of the existing hydronic piping.

OUTCOMES

The graph below shows the total energy use (in MMBtu) in the two years before the switch and the nearly four years after. Note that 2020 is not included because that was the year of construction.

CSU maintains detailed building level energy metering. By comparing steam, electricity, and natural gas use before the GSHP installation to electricity and natural gas use afterward, total energy use of the complex has dropped by more than 50%, exceeding what the energy model had projected. There are now additional portions of the building cooled in the summer that did not have cooling prior to the project. In addition, by electrifying most of the loads in the Moby Complex, as the local electric grid becomes greener, CSU moves even closer to its carbon reduction goals.

The total cost of the project was bonded at $20 million – this includes both the GSHP equipment and the upgrades to coils, fans, etc., in the building that were at end of life. On the other side of the ledger, this project removed more than $18 million worth of projects from the deferred maintenance list. As a result, the payback of the GSHP system (compared to a traditional boiler and chiller system) is estimated to be less than eight years.

The system is performing well, and the maintenance staff continue to learn the intricacies of how the heat pumps work together and adjust items like ramp speed and response times to make the equipment run more smoothly. The success of the Moby Athletic Complex Ground Source Heat Pump has prompted the university to consider other GSHP projects across campus.

Other Benefits

Beyond the energy and carbon reductions, the new GSHP system provided additional benefits to the building. There is improved temperature control in the arena where loads can swing widely based on game-day attendance. The NCAA swimming pool (that must remain at +/-1 degree of setpoint) is operating seamlessly. The only comfort challenges reported since installation exist in a small area where coils were not replaced, and the lower hydronic water temperature cannot meet the load. This was an area that was converted from a racquetball court to an office after the design was completed so the load requirements are different. In addition, taking Moby off the central steam system allowed CSU to retire approximately ½ mile of aging steam and condensate lines which created additional savings at the district heating plant.

The table below shows the performance of the system for two years before and after the GSHP was added. Water use in the facility was unchanged by the installation of the heat pump.

Moby Complex – Results following GSHP installation:

	Before	After	% Savings
Electricity [kWh/yr]	4,360,000	5,300,000	-21%
Steam [klbs/yr]	59,000	0	100%
Natural Gas [DTh/yr]	2,100	1,500	28%
Total [MMBtu/yr]	50,000	23,600	52%
Total Carbon [MTCO₂e/yr]*	3,200	180	94%

*After local grid achieves 100% renewable energy in 2030.

Charts are based on site EUI

ANNUAL ENERGY USE

ANNUAL ENERGY COST

ANNUAL WATER USE

ANNUAL WATER COST

ANNUAL PLANT ELECTRICITY USE

ANNUAL PLANT ELECTRICITY COST

IN BUILDING ENERGY USE

IN BUILDING ENERGY COST

ACROSS NETWORK USE

ACROSS NETWORK COST

ANNUAL EMISSIONS

USE

COST

CSU's Moby Athletic Complex

A Ground Source Heat Pump Solution for Colorado State University

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