Commissioning a Geothermal Heat Pump

System

Matthew Tunnard, LEED®

Horizon Engineering Associates, LLP

Presentation Goals

Describe How a Geothermal Heat Pump System WorksShow a Case Study of a ProjectShow items of Particular Concern to the Cx AuthorityShow the Pros and Cons of the System

How a Geothermal Heat Pump System Works

Geothermal FieldWhere heat is rejected to or captured fromWorks like a cooling tower

Heat PumpA simple air conditioner that can “run in reverse”

Why Use Geothermal

Ground temperature is constant and an excellent heat sinkWater Cooled Electric Heat Pumps can operate more efficiently with a constant condenser/evaporator temperatureThe Earth is a free source of energyThe system has a XXX payback

Geothermal Loop

Open LoopOpen loop takes water from aquifer and pumps it through the heat pumpNeeds very good filtration to capture particulate and keep system operating efficientlyNeed to have a large supply of water availableCan use lake water if code permits• Must worry about biological growth

Geothermal Loop

Closed LoopLoop is buried in the ground in a series of wells based on the heat load.Loop can be placed in a lake or pondSystem needs much less filtration and minimum chemical maintenance once running.Loop must be properly pressure tested before burying.

Heat Pump Operation

General OperationUses energy from the ground in a simple refrigeration cycleCan automatically switch from heating to cooling without operator input

Heat Pump Operation Cooling Mode

Heat Pump Operation Heating Mode

Case Study – Heilmann Middle School

Project Description

150,000 ft2 Middle School in Detroit MichiganGeothermal Loop under football field87 Heat PumpsOpen Protocol control system (Tridium)

HVAC System Description7 Constant Volume Energy Recovery Units (ERU) supplied fresh air make up to the heat pumpsERU’s also exhausted air from the spaces with ducted exhaustHeat pumps would return air from the space and condition it to maintain temperature set pointHP return air would mix with the fresh air make up from the ERU

Project Description Basic Floor Plan

Geothermal Loop – details

Closed systemMade up of vertical wells 600’ deepSpecial PVC PipeTwo main pumps with VFDSide stream filterAuxiliary HeaterWater filled

Geothermal Loop Operation

The pumps were sized for complete redundancyThe buildings boiler provided domestic hot water and also had a supplemental heat exchanger for the geothermal loopVFD’s maintained a constant discharge pressure

Installation Techniques

Geothermal LoopClosed Loop•Not open to air•Pipe is heat fused together•Pressure testing very important

• Pipe stretches a little

Heat Pump - Details

Sequence of OperationUnit would be in Occupied mode during set scheduleIn occupied mode compressor would cycle on/off to maintain space tempIn occupied mode the fan would remain on to provide fresh air make up from the ERU

Installation TechniquesHeat Pumps

Locations• Locate in mechanical room• Build separate closet for HP• Locate above a ceiling

Issues to be considered for location• Access to filters• Access to control valves and strainers• Replacement of compressor• Replacement of complete unit• Noise may be an issue

Commissioning Issues

Access is one of the main concerns with heat pump systems

Filters need to be changedStrainers need to be cleanedFans and compressors need to be servicedThe life of the unit is about 15 years, the entire unit may need to be replaced. This needs to be designed in.

Commissioning Issues

ControlsHeat pumps typically come with self contained unit controlsConnecting the heat pump to the BMS system can be difficult.Various companies “Open Protocol”may not be able to talk to each other

Commissioning Issues

BalancingVery important to ensure that fresh air make up is properly balanced.Transverses of small round ducts may be required to properly balance fresh air.Water balancing is critical to ensure proper loading on units.Lower operating cost

Commissioning Issues

Geothermal LoopThis is the heart of the systemThe pumps must always be running• Water must be available whenever a heat

pump starts

Should provide 3-way valves at the end of mains to always provide flow for the pumps• Attempted to use controls to maintain

minimum flow--very difficult to achieve

Pros and Cons

ProsIndependent unitsExcellent individual control• South side rooms can be in cooling while

north side are in heating.

Redundancy• Loss of a heat pump does not affect any

other spaces. No central chiller or boiler to fail.

Pros and Cons

ConsGetting the controls to work togetherGeothermal loop may restrict future site expansion• Can’t build a building on top of the loop

Initial cost of the geothermal loopReplacement of the heat pumps in 15 years.

Pros and Cons

Geothermal LoopOpen vs. Closed Loop• Open uses much less space, closed can’t be

built upon• Closed is much less susceptible to foreign

matter clogging parts• Closed has less impact on aquifers

Summary

Very simple systemVery reliableClosed Geothermal loop is virtually maintenance freeExcellent space controlNeeds to be planned for maintenance/replacementControls interface is critical