Integrating vertical & horizontal GHX's reduces construction cost

The Oasis Leisure Centre in Kalgoorlie Boulder, WA, is now heated by one of the largest closed loop geothermal systems in the State. The system is unique in a couple of respects. Even though the facility is in a hot climate, with summer temperatures as high as 46°C (115°F) and deep earth temperatures at 23.5°C (74.3°F), the building requires more heating than cooling. There are two reasons for this: 

evaporation from the pools requires make up water to be added and the evaporation cools the water, and the adjacent gymnasium uses an evaporative cooling system is very effective and efficient in the hot, dry climate. 

The second unique feature about the GCHP system is that a vertical borehole GHX is integrated with a horizontal slinky GHX that is built above it. This approach was taken for a number of reasons:

• High drilling costs ($225 / m or  $68 / foot, including grouting, pipe and tie-in) because of the drilling conditions and the remote location of the town was a strong incentive to reduce the number of boreholes.
• Expected changes to the operation of the facility over several years. During the first 1-2 years the existing roof mounted cooling systems will air condition the facility. These will gradually be replaced with air handlers connected to the chilled water system. The energy loads to the GHX will gradually become more balanced.
• The annual temperature profile of a vertical GHX is different than that of a horizontal GHX. The horizontal GHX is directly impacted by outdoor air temperature because of it's proximity to the surface. In summer the temperature of the soil 1 meter (3.28') below the surface can reach 40°C (104°F) because of the hot sun...in effect it becomes a solar collector. Winter temperatures in Kalgoorlie drop below freezing, cooling the soil. How much this impacts the temperature of the soil is directly related to the depth of the pipe.

We can take advantage of the different temperature profiles. When the system is heating, the pump on the warmest GHX module can be activated. When cooling, the pump on the coolest GHX module is selected. More importantly, energy can be transferred from the warmer GHX to the cooler. By managing the operation of the different GHX modules it's possible to manage the GHX temperature to optimize the system performance.

The operation of the GHX modules is controlled using a control package developed by Greensleeves Energy Solutions. The software uses the proprietary software that allows the input of the building hourly energy model  and hourly GHX temperature predictions and uses them to control the operation of the GHX pumps...selecting the appropriate module to target the predicted GHX temperature. The benefits include:

• A significant reduction in the size (and cost) of the GHX
• Control of the GHX temperature, eliminating long term degradation of the GHX temperature
• Greater long term reliability of the operation of the system

This project was constructed by Direct Energy Australia with design assistance from GEOptimize Inc. and Geo-Xergy Systems Inc. This project was presented at the IGSHPA Conference in Las Vegas in October, and can be seen on their website.