Recommended Practice for Selecting Design Entering Water Temperature When Sizing Ground Heat Exchangers
The overall efficiency and performance of a geothermal system for heating and cooling buildings is highly dependent on the design and the size of the ground heat exchanger (GHX). Improperly sized ground heat exchangers can be either (a) oversized for the design load, resulting in excessive capital cost of the system with marginal improvement beyond published efficiency ratings, or (b) undersized for the design load, which results in insufficient heat transfer to meet the load, inefficient heat pump operation, increased operating costs, and unsatisfied customers. An undersized GHX is frequently the root cause of problematic ground-source heat pump systems, often because the designer/installer wanted to reduce the cost of the Earth coupling.
Selection of an appropriate Entering Water Temperature (EWT) to the ground-source heat pump when designing a geothermal system will address the issues of the undersized GHX as long as the system is properly constructed and commissioned. When operating in heating mode, which is the dominant mode of operation for most geothermal systems in New England, the EWT must be lower than the average ground temperate in order to conduct heat towards the GHX and into the heat pump. However, if the EWT is too low, the heat pump is over-worked and operates less efficiently.
The International Organization for Standardization (ISO) has specified standard EWTs for testing the performance of heat pumps in ISO 13256 for both Open- and Closed-loop systems. This internationally recognized standard is summarized in Table 1 for both open-loop and closed-loop ground-source heat pump systems operating in heating and cooling modes. The New England Geothermal Professional Association (NEGPA) formally endorses the following standard EWT design temperatures for various popular geothermal systems in New England, which also includes the Standing Column Well geothermal system which are also common in the New England area.
Table 1 – NEGPA Endorsed Design EWT Standards
|Open-Loop EWT||Standing Column EWT||Closed-Loop EWT|
|Heating Mode||≥ 10 °C (50°F)||≥ 7 °C (45°F)||≥ 1 °C (34°F)*|
|Cooling Mode||≤ 15 °C (59 °F)||≤ 18 °C (65°F)||≤ 25 °C (77 °F)|
NEGPA recognizes the need for flexibility in design and operation of geothermal systems. However, the rationale for and effects from any deviation from these design standards should be clearly explained to the owner/operator of a system being designed.
*ISO 13256 is based on 0 °C (32°F), but NEGPA recommends 1 °C (34°F)