By Lane Theriault
The way buildings are being constructed is changing quickly, largely out of a desire for a greener built environment. Municipal building codes are now becoming more stringent, resulting in greater energy efficiency. As a result, developers need to incorporate sustainable technologies to meet these demands.
Current building codes have an increased focus on energy efficiency. If a developer is looking to meet these metrics, there is no better place to start than with the heating and cooling plants within buildings. Aside from being one of the most energy-intensive parts of a building, it is where almost all of the onsite greenhouse gas (GHG) emissions are created (Figure 1). This is an area environmentally focused developers must pay attention to.
To implement a geothermal energy system, it is important to first understand what exactly it is. The simplest way to describe geothermal energy systems is to imagine the ground as a large heat battery—with a one-year charge—that cycles this heat in and out of the earth. All geothermal systems rely on heat pump technology. The pump removes heat from the building and sends it into the ground. The heat travels through a series of fluid-circulating pipes in contact with the ground, transferring the heat from the fluid to the earth under the building. In the winter, the flow reverses, and the heat is extracted out of the ground to warm the building.
This process makes geothermal energy efficient. It functions without creating excess energy to heat and cool the building, and instead moves the existing energy within the building over the course of the year.
Setting up the plant
A geothermal heating and cooling plant can be set up in two ways:
- full; or
Hybrid systems incorporate conventional equipment (e.g. the boilers and cooling towers) to supplement the geothermal field while the full assembly is usually absent of these.
Hybrid systems can usually be more capital efficient providing more return on investment (ROI). This is due to different energy demands for buildings. Most HVAC plants are sized to meet the highest heating and cooling loads of the year, also known as the peak capacity. However, those peaks are very few and far between during most of the year and only a small fraction of that capacity is used.
In general, there is a dichotomy between geothermal and conventional technology. Geothermal boreholes are costly to drill, but once in place, are inexpensive to operate. Conventional HVAC equipment is the opposite, being less expensive to install, but costly to run, maintain, and input energy. Hybrid systems take advantage of this by essentially splitting the work of the plant into two pieces:
- the peaks; and
- the baseload.
In designing a system with geothermal handling or the baseload, one can justify the higher initial capital costs because of its high utilization and low operating cost.
Conversely, it is difficult to justify the high capital cost of meeting peak load capacity with a geothermal system when one could just install inexpensive conventional equipment that runs infrequently. In Canada, the optimal ratio of hybrid geothermal to conventional is about 30 to 70 per cent respectively, although other factors can affect those numbers. Figure 2, provided by Reshape Strategies, an advisory and development services provider for energy and infrastructure, illustrates results from a feasibility study on hybrid systems.
Advantages and disadvantages of a hybrid system
Aside from the capital efficiency feature, hybrid designs also have several other advantages over their full geothermal counterparts (Figure 3). An important factor is the ability to easily balance the energy loads over the year. The idea that the ground acts as a heat battery implies the same amount of energy coming in and out of the field is needed every year, or else the field could get too hot or too cold, worsening its overall efficiency. Hybrid systems have an embedded ability to prevent this from happening. If the seasonal balance is unusual, the conventional system can be run for a longer period of time to restore the equilibrium.