Blog

December 8, 2015

Geothermal Systems

Recently our Vice President, Edward Trueman, gave a lunch and learn seminar outlining the benefits, uses and commissioning guidelines surrounding geothermal systems. Being a clean and renewable energy source, it is important to educate people on geothermal energy and promote its use over fossil fuels and the damaging greenhouse gases they produce.

How Geothermal Systems Work

Geothermal systems use the ground as a heat exchanger.  A heat source during the heating mode and a heat sink during the cooling mode.

In the heating mode, the geothermal system circulates fluid, typically a glycol solution, through tubing in either a vertical or horizontal arrangement.  The temperature of fluid entering the ground is typical lower than the ground temperature and heat energy is transferred to the fluid from the ground.

In cooling modes, the temperature of fluid entering the ground is typical higher than the ground temperature and heat energy is transferred from the fluid to the ground.

A geothermal system is typically used with water to water or water to air heat pumps.  The geothermal system replaces the cooling tower and heat injection from a boiler plant used to maintain the condenser water temperatures for the heat pumps.

Advantages of Geothermal Systems

1. The biggest advantage of geothermal systems is that they are renewable. Barring gross overuse, the Earth will continue to provide energy.
2. A geothermal system requires only the energy required by the pumps to run. In geothermal power stations, the energy the power station itself generates can be tapped for this, allowing for no outside power/fuel source.
3. There are significant cost savings associated with geothermal systems. No fuel is used to generate or transport power, making it inexpensive to run, except for the electrical energy required by the pumps and heat pumps.

Disadvantages of Geothermal Systems

There are few, but they are things to consider when deciding if a geothermal system is the solution to your energy requirements.

1. The composition and properties of the local soil and rock affects the heat transfer rates, which affects the size and design of the ground loop. Vertical loops may be required over horizontal loops if the soil is too shallow to trench, it is an area with extensive hard rock. Or the field space is restricted.
2. They generally have a higher initial cost than conventional heating systems.
3. Secondary / back-up heating systems are required in colder climates.
4. Geothermal energy must be used in the surrounding area, as it cannot be easily transported like wood, coal or oil.

Commissioning of Geothermal Systems

Proper commissioning of geothermal systems, or ground heat exchangers, should be done in a specific chronological order. The first step is the process of flushing the system. This entails passing clean water through the ground heat exchanger to remove foreign debris and silt, leaving clean water. Ensure each header and main line are flushed individually. Also, when planning the flushing process, flush in an order that allows previously flushed sections to remain clean as others are completed.

The second step is purging, where clean water is flushed through all sections of the ground heat exchanger at a minimum of 2 ft/s to remove all air from the piping.

The third step is pressure testing. Piping is hydrostatically pressure tested to ensure there are no leaks anywhere in the system, as repairs on geothermal systems once they have been buried is extremely difficult and prohibitively expensive.

The fourth step is dynamic flow testing, where clean water is run through the ground heat exchanger at the proper design flow rates. This is to check for any possible flow constrictions in the piping and to ensure all parallel branches are passing water.

The fifth step is balancing, where balancing valves and control devices are adjusted to ensure design flow rates are achieved through each section.

The final step is antifreeze charging, where antifreeze is introduced to achieve the design concentration. This is to protect the piping and ground heat exchanger from freezing at low operational temperatures.

Scheduling is an important part of commissioning a geothermal system as there are short windows of opportunity where certain tasks can be accomplished. To ensure there are no construction delays or compromised commissioning activities due to a lack of time, the Commissioning Consultant should meet with the Project Coordinator and the GXH Contractor to determine appropriate time frames, which should be included on the overall construction schedule. This will ensure a smooth process and a properly commissioned system.