Renewable energy technologies are typically more costly to install than conventional ones, though scaling up of technologies has resulted in a significant drop in costs across most areas of renewables.

Return on investment is a major concern for most clients though – more so with commercial developers than institutional clients. Much of the work done in the geothermal heat pump industry is to find methods and techniques that either reduce construction cost or improve efficiency, or both.

Investors are financing the installation of ground heat exchangers for building owners, or even developing utilities based on these systems. Yet few engineering design firms are comfortable designing the ground heat exchangers (GHX) needed for such systems. Designing a GHX forces a designer to work outside his or her area of expertise.

Few mechanical designers are knowledgeable about the geology, drilling and excavation skills needed to design a GHX.

Designing a GSHP system does not fall into the area of expertise of a geologist or driller either, and very few have experience in mechanical system design, energy modelling or hydronic system design needed to design either a GHX or a mechanical system to go with it.

Ultimately, a mechanical system designer must take responsibility for the design of the GHX, since it is integral to the operation of the system.

Ground heat exchangers

Designing a GHX requires a different approach than that used in designing a conventional mechanical system. If a building is connected to the gas or electrical grid, energy can be considered to be infinite (as long as the utility bill is paid). If the gas line is large enough to supply enough heat on the coldest day it will heat the building the rest of the year.

It's the same for the cooling. As long as the cooling tower can dissipate all of the heat on a design cooling day, it will operate adequately year round.

Designing a GHS is more like designing a heating system for a project in a remote community where oil can be delivered once a year. The fuel storage tank must be large enough to last the winter, but not so large that the construction cost is prohibitive.

A GHX is similar to designing an energy storage tank from which heat can be drawn or rejected to. It's not perfect, however, since some energy leaks in and out to the ground around it. Heat also dissipates in winter to cold outside air, or is added when solar energy hits the ground. Energy transferred to and from the building has a much greater impact on the temperature of the GHX. Because the GHX operates like a storage tank, it is critical that the amount of energy added to – or removed from – a building over the year is calculated with a high level of accuracy.

Some buildings, because of their location, orientation, construction and use, require more heating than cooling over the year. They are considered to be heating dominant. Others require more cooling and are considered cooling dominant. If the annual heating and cooling energy loads in a building are fairly equal, the building energy loads are considered to be balanced.

In cooling dominant buildings energy must be dissipated to the earth around the GHX or to cool air above the ground. If not enough energy is dissipated to the earth around the GHX, the temperature of the fluid circulating through the GHX and the heat pumps will gradually increase and affect the efficiency of the heat pumps. To prevent gradual warming a larger GHX is needed.

Alternately, if the building is heating dominant, the GHX will gradually cool (unless there is enough energy input from the earth around the GHX or from solar energy hitting the earth). The GHX must again be larger to allow enough energy to leak in from the ground or the sun. But if the annual energy loads to and from a building are fairly balanced, a smaller GHX can be designed because there is less reliance on energy from the earth around the GHX, solar energy input or heat dissipation to the outside air.

So why is the temperature of the earth around the GHX important? Heat pumps connected to a GHX are designed to work efficiently within specific temperature ranges. As the temperature of the GHX approaches the limits, the heat pumps operate less and less efficiently, and will eventually stop working.

System improvement drives down costs

The cost of the mechanical equipment for a GSHP system inside the building is similar to that of a conventional mechanical system. After all, the compressor in a rooftop unit and a geothermal heat pump is identical. The cost of the distribution systems is identical. The cost difference therefore is found mostly in the construction of the ground heat exchanger. If the cost of building the GHX is reduced while improving the efficiency of the system, the payback to the building owner improves.

The key is to work with the building owner and design team. If the heating and cooling loads are better balanced, the size and cost of the GHX is reduced, the efficiency is improved and long term performance of the system is ensured.

Designing a GSHP system must begin with a solid understanding of the building energy model. Is the building heating or cooling dominant? This can only be determined by building a computer simulation of the building. The geometry and occupancy of each room in the building must be described in detail. Information needed includes:

• Construction of each room – How much insulation is specified? What is the shading factor of the glass in the windows? What is the orientation of the glass? Does another building shade the glass part of the day?
• Occupancy schedule – What time of day will the room actually be in use, and by how many people?
• Lighting schedule – Will the lights be on twelve hours per day, or will occupancy sensors be used? Will other electrical equipment have an impact on the room?
• Ventilation schedule – What is the ventilation rate for the room? Is heat recovery from the exhaust air be specified? Is the ventilation operating 24/7, or is it scheduled, or will CO₂ sensors be specified to provide fresh air only as required?

The building construction, orientation and scheduling is then overlaid with hourly weather data for the building location to develop an accurate building simulation. The more accurate the information that is input, the more accurate the building simulation becomes.

After preliminary building loads are calculated there are often opportunities to affect the building heating and cooling loads. The energy model can be easily adjusted to provide feedback to the team. If cooling loads are dominant, is it possible to change the window specifications, or specify a white or green roof rather than a traditional dark coloured roof? Can the lighting design be adjusted, or occupancy sensors specified to reduce the lighting loads? Can excess heat energy be used to preheat domestic hot water to divert some heat away from the GHX?

If the building is heating dominant, can more efficient heat recovery be specified for the ventilation air? Possibly less reflective glass will improve solar gains into the building and reduce the heating loads. Can heat from the elevator equipment, computer server rooms or restaurant refrigeration equipment be rejected into the GHX rather than to an air cooled condensing unit?

In the early stages of the design process there are often opportunities to make recommendations to the owner and design team to balance the energy loads to and from the ground heat exchanger based on the preliminary energy model. An example of how an energy simulation can impact the cost of owning and operating a typical retail store is shown on page 23. The store is approximately 120,000 square feet (11,150 m²) in size and is located in the cold Canadian prairies:

• The first GSHP system is based on the standard building. Geothermal heat pumps and a GHX simply replace gas rooftop heating / cooling units;
• The second GSHP system shows the impact of heat recovery from the exhaust air, with a reduction of the heating load;
• The third GSHP system illustrates how careful lighting design helps balance the heating and cooling loads of the building.

The cost of adding exhaust air heat recovery and high-efficiency lighting is added to the total building cost as are the savings realised in the cost of installing the GHX. Additional incentives are available to the building owner because of improved energy efficiency, and the cost of the GHX is reduced. The overall energy cost savings are increased from $17,300 annually to $41,700 annually, reducing the simple payback to a very acceptable 3.6 years.

Detailed energy simulations such as those demonstrated in this research provide the owner with the information needed to create a better building, lower energy cost and reduced construction cost.

About:

Ed Lohrenz works for Geo-Xergy Systems Inc, which specialises in the design and implementation of geothermal heat pump systems.

Renewable Energy Focus, May/June 2011.