The Other Sustainable Option—Geothermal Heating and Cooling
Tetra Tech’s High Performance Buildings Group reflects on the benefits of implementing geothermal energy as a sustainable solution.
When most people hear the term geothermal, they usually think of large-scale power generation. But geothermal is a viable, clean heat source that can provide substantial operational cost savings for many precinct-level developments—even in the middle of a busy city.
Geothermal has long been considered a viable way of generating power, for individual building or small precincts. However, the start-up investment for a facility capable of powering a city is considerably greater than other clean energy sources, such as solar or wind. A key difference is that geothermal also offers a reliable, cost-effective and unobtrusive solution to the heating needs of precinct developments and public facilities.
Associate Director Jeff Dusting, with Tetra Tech’s High Performance Buildings Group, has worked on several geothermal heating projects, and believes that there can be significant operational cost benefits as well as an improved occupant experience with a localized geothermal system. “In the right project, geothermal can lower operational costs, as well as providing a near neutral carbon footprint for the precinct,” he says. “This operational saving improves cash flow for the owners, as well as lowering the expenditures of tenants and users of the facility. One recent example is Scarborough Pool in Perth. The geothermal system is so efficient that there is no requirement to use solar blankets or boilers for the outdoor pool. This also saves on staff time, keeping costs down even further.”
What exactly is geothermal?
Geothermal is essentially using the natural heat found deep underground to power or heat a building above ground. This can be ambient heat from over a kilometer underground, or tapping into water sources that are heated by underground activity, such as geysers or hot springs.
Geothermal power generation requires temperatures high enough to boil water, requiring very deep wells and pipes. This increases the initial cost considerably; however, the temperatures for heating a building to comfortable levels are substantially lower. This means that the depth and complexity of the system can be reduced, making it viable for much smaller scale projects.
Direct versus indirect heat transfer
Geothermal heat transfer comes in two main types: direct and indirect.
Indirect uses heat from deep below the surface to heat a refrigerant, which is then used to warm up the environmental systems. The Scarborough Pool in Perth uses this type of system, providing with a constant supply of warm water to keep the pool heated and comfortable all year round.
Direct geothermal energy uses pipes that are sunk just a few meters into the ground to extract heat in winter for heating (through direct heat transfer through the metal, or via a refrigerant pumped through the pipes) and to sink the heat in summer, cooling the building.
Some buildings can utilize both systems simultaneously.
Tetra Tech designed one of the first geothermal projects in Manchester. The First Street Site is a 250,000-square-meter (2,690,97-square-foot) mixed-use development that utilizes a ground source heat pump that taps into the heat capacity of the ground, which at 12 meters below the surface remains fairly constant. This improves the efficiency of the building’s heating and cooling plant. Closed loop boreholes effectively use the ground as a heat “sink” throughout the year whereby energy from the ground is either rejected (cooling) or absorbed (heating) into the system.
The borehole array can then be connected to a series of water-cooled variable refrigerant flow units, which provide both the heating and cooling for the building. This complements the other boreholes that were installed at a depth of 150 meters. These boreholes have the potential to provide up to 300 kilowatts of peak heat rejection, representing over 20 percent of the cooling demand for the building. In heating mode, this arrangement can deliver 188kW of heat absorption, as well as keeping the “ramp up” costs down when heating or cooling is turned on for the day.
“These geothermal systems don’t generate electricity, but they save a lot of it in the building,” says Jeff. “Energy use in buildings accounts for about one quarter of Australia’s greenhouse emissions, and over half of this is for heating and cooling. With a geothermal system, we can slash the carbon output to basically nothing, and minimize the ongoing heating and cooling costs for the project,” he adds.
Tetra Tech has worked on several other geothermal projects around the world, including one of the first geothermal projects in Manchester, England.
Number One First Street is a mixed-use building, with tenants that include Manchester arts organizations, Cornerhouse and Library Theatre Company, a four-star Melia hotel, 700 space multi-story car park, nine new restaurant/café units, student accommodation, and a new public square.
The project used ground source heat pump systems to access the heat capacity of the ground, which at 12 meters below the surface remains fairly constant to improve the efficiency of a building's heating and cooling plant. Closed loop boreholes effectively use the ground as an energy “sink” throughout the year whereby energy from the ground is either rejected (cooling) or absorbed (heating) into the system.
The ground source heat pump and borehole solution take advantage of the moderate temperatures in the ground to boost the efficiency of the mechanical systems and reduce the running costs of heating and cooling plant.
The biggest advantage of such systems is gained when heating and cooling is carried out throughout the year, as the heat rejected in summer is stored in the ground and used for heating in winter.
“There’s this school of thought that geothermal is only for new buildings,” Jeff says. “That’s just not the case. There are many ways to integrate a cost-effective geothermal solution into an existing building, even in a major metropolitan area.”
Geothermal for Pools
“Pools and aquatic centers represent a large number of our geothermal projects,” says Jeff. “The heat requirements of these facilities make geothermal a very attractive option, with a very short payback period.”
By using renewable energy to heat the water, rather than traditional gas boilers, hundreds of tons of carbon are saved, as well as significant operational costs. At the Scarborough pool, the City of Sterling estimates that the geothermal system saves them around half a million dollars in heating costs per year, a saving that allows the council to put funds to much more practical use.
As precinct-level power generation and services sharing becomes more popular, geothermal will also form an integral part of providing precinct-level sustainable solutions.