By George Peters
Well I think we can all agree that there is nothing like a poor economy to turn everyone’s focus toward cost reduction. Toss in rising energy costs and government stimulus programs and it is only natural to start looking at your buildings HVAC systems for operating cost savings. These combined systems (Heating, Ventilating, and Air Conditioning) represent the largest building related operational expenses in a typical commercial property today.
In broad terms there are three ways to reduce the HVAC operating expenses of any building. First, decrease the unit cost of energy by negotiating or shopping for better rates or for alternative sources of energy (i.e. switching from Electric Heat to Natural Gas). Secondly, one can modify their habits to lower energy requirements. For instance altering internal space temperature set points (lower heat in winter and less cooling in summer), or adjusting set points when the space is not occupied. Finally, a facility owner can upgrade their current HVAC systems to a more efficient arrangement. One such system is a Geothermal System. This third point is the essential element of this commentary’s discussion. It is the overall efficiency of geothermal systems, along with other unique features, which makes them attractive to the Commercial Real Estate Market.
At this time I should explain and define just what is meant when discussing a “Geothermal System”. The word “Geothermal” defined; “relating to, or utilizing the heat of the earth’s interior”.
A Geothermal Heating and Cooling System provides space conditioning — heating, cooling, and humidity control. Geothermal Heating and Cooling Systems work by transferring heat to and from the earth. Every Geothermal Heating and Cooling System has three major components: a geothermal heat pump to exchange heat between the building and the water in the earth connection, an earth connection for transferring heat between the water and the ground, and a distribution subsystem for delivering heating or cooling to the building. “Geothermal” encompasses a number of system types within the commercial market. Systems can be in an “opened” or “closed” loop, central plant, or distributed units. The common element among them is that they all use the energy stored in the earth.
Open loop systems are the simplest. Used successfully for decades, ground water is drawn from an aquifer through one well, passes through the heat pump's heat exchanger, and is discharged to the same aquifer through a second well at a distance from the first. Open loop systems can be problematic though, depending on the water quality and quantity of the area aquifer. For these reasons closed systems were developed.
Closed loop systems are now becoming the most common. When properly installed, they are economical, efficient, and reliable. Water or a water and antifreeze solution is circulated through a continuous buried pipe. The length of loop piping varies depending on ground temperature, thermal conductivity of the ground, soil moisture, and system capacity.
In either system (closed or open) heat is transferred either from the loop to the space, or from the space to the loop. In our region at a depth below four feet, the ground temperature stays a constant 50 to 55 degrees all year round. Therefore the water temperature of the loop is more stable than seasonal air temperatures so that in winter a geothermal system transfers the heat of the earth into the building. During the summer, the system transfers the heat from indoors and moves it back underground. This energy transference is the fundamental concept that one must fully grasp to understand all water sourced heating and cooling systems.
Geothermal equipment uses heat pump technologies to both heat and cool air. The term “Heat Pump” has left a negative impression in many people’s minds in areas that have a significant heating season. This is the result of experiences with air-sourced heat pumps. An air source heat pump uses the outside air as the source to collect or reject energy. As air temperature drops less energy is available for collection which results in less heat production and lower discharge air temperatures yielding poor comfort. Geothermal heat pumps use the earth’s constant energy instead of the air. This means that a Geothermal heat pump can deliver warmer air in the winter and not experience the “cold draft” commonly associated with the term “heat pump”.
The geothermal heat pump is typically packaged in a single cabinet, and includes the compressor, loop-to-refrigerant heat exchanger, and controls. Systems that distribute heat using ducted air also contain the air handler, duct fan, filter, refrigerant-to-air heat exchanger, and condensate removal system for air conditioning. In commercial installations, it may be hung above a suspended ceiling or installed as a self-contained console.
Earlier we mentioned that Geothermal Systems can be “Central Plant” or “Distributed Units”. Examples of these two systems are the easiest way to illustrate what we mean by these phrases. A “Central Plant” system contains a large central unit, such as a Geothermal VAV Air Handler (Variable Air Volume) and zone controls, such as VAV boxes to regulate comfort (zoning). In a “Distributed Unit” system each area or zone has a separate geothermal heat pump.
What does all this education mean to you? How can you use this knowledge to help lessen the impact of future energy cost increases and also help limit your facilities Carbon Footprint at the same time? First, realize that any commercial building can be upgraded to a geothermal system, but also recognize that some buildings are far easier and much less expensive to retrofit than others.
Let’s look at two common scenarios:
For our first example we will consider a three-story office building that currently has packaged rooftop VAV systems (one unit per floor) and hot water re-heat VAV boxes. Each rooftop VAV unit has gas heat for morning warm up and unoccupied heating (times when the entire building calls for heat). The hot water reheat is generated by gas fired boilers located in a first floor mechanical room. This existing system provides a good level of comfort (zoning) and quiet operation. It does “waste” some energy though through the gas fired re-heat system. These systems typically run in the Air Conditioning (Cooling) mode during occupied hours. Comfort is maintained by “re-heating” the “Air Conditioned” air in those zones that call for heat or do not need the air cooled.
To retrofit this building with a Geothermal System we would first need to access a geothermal source. As previously noted a Closed Loop system is typically used. In our region we typically install a vertical bore loop system.
This is a series of holes that are drilled into the earth. The vertical bores are generally drilled 15 to 20 feet apart to depths of around 400 feet. Tubing is then inserted into these holes to create the earth coupled “closed loop” system. The tubes within each bore are then piped into common headers and routed to the building in 4 foot deep trenches. The header piping will be routed into the first floor mechanical room. This whole grid of bores and tubing is usually located under the parking area, but any open, reasonably flat area will suffice.
Now that a Geothermal Source has been accessed the building in our example will need some new mechanical equipment. The existing roof top units will need to be replaced by packaged rooftop geothermal heat pumps. The existing duct systems will remain. The gas fired boiler will be replaced by water to water geothermal heat pumps. The existing hydronic pipe system will remain. Both the roof top units and the water to water units will be connected to the geothermal piping loop with new pumps located in the mechanical room.
This new “Central Plant” system will look and perform like the original system. It will provide good comfort levels with little noise. This new system though will not waste energy for re-heat. It will re-use the waste energy from the air conditioners. The entire system will operate at a much higher efficiency level with the geothermal source of energy. You will note that nothing was reconnected to the gas utility.
For our second example, a “Distributed Unit System” will be used, let us consider another three-story office building that currently utilizes multiple horizontal water source heat pumps above the ceilings, gas fired boilers located in a first floor mechanical room, and a cooling tower situated on the roof of the building. This existing system (like the VAV system) also provides a good level of comfort, but it is not as quiet due to the heat pumps being located above the ceilings. It does have an efficiency advantage since re-heat is never needed. The individual units operate independently of one another, providing heat or cooling as needed.
Following the process from our VAV retrofit, the first thing we need to access is a geothermal source, and in fact the process is exactly the same. We need to create a vertical bore closed loop. It doesn’t matter what equipment the earth is coupled, with the earth is simply a source of energy, much like gas or electricity.
Now for the equipment; the existing heat pumps will be replaced by extended range water source or ground coupled heat pumps. The existing duct systems will remain. The existing gas fired boilers and existing cooling tower will not be needed. They can be removed or abandoned in place. The existing system piping and pumping will need to be slightly reconfigured and connected to the new geothermal source. Again this new “Distributed” system will look and perform like the original system. In fact this new arrangement looks identical, but it will operate at a much lower cost due to the geothermal energy source.
These are fairly simple examples used to illustrate the retro-fit process. All buildings are different (structure, location, use) and the solutions for each will be different. The solution for your building should be determined with the help of qualified professionals such as a design-build Mechanical Contractor or a Professional Engineer who works in tandem with knowledgeable contractors. In either example we did not address power wiring, control systems, and other system related equipment. The existing power wiring in each case will be very similar to what is required for the retrofit, but some costs may be incurred for new pump circuits. The controls systems are an entire subject of their own and too complex and varied to even begin discussing in this article. Finally just because an existing component may work with the new system does not mean that one should not consider replacing it when retrofitting the facility. The component might be near the end of its useful service life or even be a potential source of additional energy savings. For example, a new pump may be far more energy efficient than an older model, much like a new car gets higher mileage than one built thirty years ago.
Two last points to consider when upgrading or retrofitting your current HVAC system to a Geothermal System; first, don’t skimp on the efficiency of the new heat pumps. Install good high quality, high efficiency equipment in order to maximize your energy savings. Secondly, be wary of going to a total geothermal system if your current building operates in the Air Conditioning (Cooling) mode most of the time. Buildings that operate around the clock, or that have large data centers may be better served with a hybrid system — meaning a geothermal system that also uses a cooling tower for excess heat removal.
In conclusion, Geothermal Systems are efficient, environmentally-sensitive, comfortable, and economical. Operating savings often provide paybacks of considerably less than five years— sometimes even less than two years.
The key principle is that geothermal heat pumps use electricity to only move heat; they do not generate heat by the burning of fossil fuel or using electric resistance elements. No existing space conditioning technology offers greater comfort, economy, or environmental benefits than the geothermal heat and cooling systems now available.
About the author:
George Peters is the senior Mechanical Engineer for the H.B. McClure Company. He has held this position for over 15 years and has been responsible for designing and building over 400 facilities that utilize geothermal energy and water sourced heating and air conditioning technologies. H.B. McClure is a leading mechanical contractor in Central Pennsylvania and has been providing outstanding service to the mid-state since 1914. George can be reached by phone at 717-232-4328 or firstname.lastname@example.org.