Aqualinc: Groundwater as an energy source
By Dr Helen Rutter and Julian Weir, Aqualinc
Christchurch is underlain by a series of confined aquifers ranging in depths from five metres to greater than 200 metres. These aquifers are highly productive, with yields in excess of 100 litres per second possible from single wells, and the aquifers flow freely under artesian pressure in many areas. This abundant source of water makes the Christchurch aquifers ideal for supporting ground source heat pump (GSHP) systems.
The principle of GSHP systems is similar to air-sourced heat pump systems, except that groundwater (abstracted from onsite wells), rather than air, is passed through heat exchangers to extract or reject heat energy as required for heating or cooling. The water is discharged to a soak pit or reinjected into bores, returning it to the ground. This in turn reduces or increases the groundwater’s temperature. In an urban environment, injection is usually the preferred option.
The advantage of GSHP systems lies in their use of groundwater where the water temperatures usually remain in the 12-14°C range. This compares with air temperatures that, in Canterbury, vary between below freezing and over 30ºC. As a result, GSHP systems are more energy-efficient. At certain times of the year, groundwater can be used directly for cooling, with no need for heat exchangers.
Christchurch aquifers were already being used for GSHP systems prior to the earthquakes. Examples include the University of Canterbury, where groundwater was used for cooling laboratories and lecture rooms, and Christchurch International Airport, which has the largest GSHP system in Christchurch. The Airport’s older diesel and LPG boiler system was replaced in 2011, and now comprises two 1.5 MW and one 600 kW GSHP systems for heating and cooling.
New systems have been recently developed as the city centre rebuilds post-earthquake. These include systems at the Arts Centre, Bus Interchange, new Central Library, Justice Precinct, and King Edward Barracks. The use of groundwater for such systems relies on being able to abstract groundwater from one aquifer layer and inject it into a different layer: in this way, recirculation of the warmed or cooled water, which reduces system efficiencies, can be avoided. However, some schemes have found that while abstracting water is straightforward, injection poses more of a concern due to problems with shallow groundwater mounding. A greater understanding of the three-dimensional variability of the permeability of the aquifers under Christchurch will enable a better understanding of where such systems might and might not be viable.
While abstraction and reinjection GSHP systems do not usually result in a net removal of groundwater from the aquifer system as a whole (the injection rate is usually the same as the abstraction rate), abstraction of groundwater causes a cone of depression in the source aquifer and the subsequent reinjection results in mounding in the destination aquifer. These effects may overlap, depending on the amount of leakage between the two aquifers. Furthermore, adjacent GSHP systems in the same aquifer will also interfere with each other. The same occurs thermally, whereby injecting water that is warmer or cooler than the receiving groundwater will result in a thermal plume that will propagate and dissipate away from the injection well, affecting the efficiency of down-gradient GSHP systems. Numerical modelling of the groundwater system can help us understand and quantify both the hydraulic and thermal effects in groundwater from operating GSHP systems. This is achieved using solute transport models where solute transport properties are exchanged for heat transport properties.
Some shallow GSHP systems do not abstract water, but simply pass fluid through pipes in the ground that is then warmed or cooled by the ambient groundwater temperature. In these cases, thermal transport modelling alone is sufficient, as there is no change to the hydraulic response of the groundwater system. However, in systems where groundwater is abstracted and then reinjected (either in the same layer or in a different layer), it is important to model both the hydraulic and thermal responses simultaneously, as the change in hydraulic response contributes to the propagation of the thermal plume.
Understanding the effects of GSHP systems on the groundwater system and on other nearby GSHP systems is key to making these systems a truly efficient and sustainable option for large-scale heating and cooling.
For more information about groundwater in New Zealand, please see Aqualinc.
Date posted: 10 December 2018