Modeling Time Period Information Heat exchangers potentially can change the ground temperature- especially when the cooling and heating loads are unbalanced. In such cases, it can take several years or more before the ground temperature stabilizes. With Ground Loop Design, designers can change the modeling time period (1 year, 5 years, 10 years, etc) and then calculate the heat exchanger length for that time period. For example, in an unbalanced system the soil temperature will be changing over time and therefore, the loopfield length will be changing (most likely increasing) over time as well. If a system is unbalanced, designed with a one year modeling period and then installed, it is possible that the heat exchanger will not be long enough to function effectively in the soil conditions 5 years out and the system will fail. Hence, in some cases it is worthwhile to model over longer time periods and see when and at what length the heat exchanger length stabilizes (it stabilizes when the ground temperature stabilizes). For residential systems, ground temperature changes are uncommon since the systems are fairly small and usually relatively balanced. A number a factors can influence the time it takes for the ground temperature to stabilize including: load balances/imbalances, water flow in the soil (flowing water can transfer heat quickly), and borehole depth. For shallow boreholes, a significant percentage of the borehole interacts with the atmosphere/ sunlight and hence has some of the characteristics typically associated with a horizontal system. Such systems tend to have reduced or no soil temperature changes since the soil can exchange heat with the atmosphere. For such systems, short modeling periods oftentimes can be used. As the boreholes get deeper, a greater percentage of the borehole is not affected by sunlight/the atmosphere and hence, long term soil temperature changes become more common. As with all engineering designs, it is ultimately the responsibility of the designer to choose the appropriate design inputs. |