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Estimating efficiency

A system for finding the total length of a ground source heat exchanger
is too simple to evaluate the global efficiency of the future system.

A system for finding the total length of a ground source heat exchanger is too simple to evaluate the global efficiency of the future system. The 70/90 per cent guideline is stated in CSA C448.2-02 Clause 10.3.1.

The following text, which is a part of one of our articles (co-written by Denis Tanguay) may provide better comprehension of this clause.

Design and installation of residential GSHP systems in Canada are governed by CAN/CSA C448.2-02 – Design and Installation of Earth Energy Systems for Residential and Other Small Buildings (hereafter C448). Article 10.3.1 of this standard specifies that: The heat pump(s) shall be sized in such a way that their rated heating capacity, at the minimum entering liquid temperature (0 C  or 32 F for closed-loop systems and 10 C or 50 F for open systems), is not less than 70 per cent of the building’s design heat load. The combined output of the heat pump and any supplementary heat shall be equal to or greater than 100 per cent design, but the heat pump shall not exceed 105 per cent of the design heat loss.

Note: The intent of this clause is to ensure that the heat pump supplies more than 90 per cent of the building’s annual space heating energy load.

The standard requires a minimum heating capacity for the heat pump relative to a building’s need. However, there is nothing to guarantee that the unit will be able to provide it because the system is not referred to as a whole: standard C448 is silent about the minimum entering water (EWT) temperature of the loop in heating mode. According to C13256, the ground source heat pump (GSHP) unit EWT for a closed loop is 0 C and 10 C for an open loop. But there is no way to guarantee this temperature. Standard C448 does not require any minimal water temperature for system design, and rightly so (no minimum EWT can be required since EWT is actually dictated by a variety of factors, including financial parameters). The minimum heat pump capacity of 70 per cent in standard C448 aims at ensuring that the unit will provide 90 per cent of the building’s annual space heating energy load. Note that this 70 per cent rule overestimates the minimum capacity required to provide the 90 per cent.

Standard C448 does refer to standard C13256, which mandates a specific minimum co-efficient of performance (COP) for the unit, according to the type of GSHP system installed. C13256 says:

• Groundwater heat pump (EWT = 10 C), minimum COP = 3.6
• Ground loop heat pump (EWT = 0 C), minimum COP = 3.1
• Water loop heat pump (EWT = 10 C), minimum COP = 3.6

Here again, there is nothing to guarantee that the unit will not have a lower field performance since the EWT can be below those established in C13256.

In reality,  as many as 75 per cent of GSHP systems in Canada will function with EWT below 0 C in intense cold. The heat pump unit ratings are not representative of the conditions in Canada. Industry discussion suggests current heat pump ratings are based on theoretical EWT in an average climate. Ground temperatures are higher than in Canada.

Adjustment based on ISO 13256, C13256 was never appropriately adapted for Canada. Most residential design software on the market is not able to address minimum requirements set in standard C448. In the Canadian Geoexchange Coalition (CGC)’s staff and member-reported experience, current design tools are often too simplistic, not all user friendly, and most do not consider important design parameters, such as grout properties, borehole diameters or thermal interference between boreholes. In addition, the choice of regions is limited.

To address these weaknesses, CGC staff developed an innovative and more advanced computer assisted design and analysis tool (GeoAnalyser). For closed vertical GSHP systems, this tool uses the commercial system design method proposed by Kavanaugh and Rafferty (1997). The software produces an estimated Seasonal Coefficient Of Performance SCOP based on the bin method, and the BIN calculation is based on the SCOP estimation method presented in by the American Society of Heating, Refrigerating and Air-Conditioning Engineers in 2005.

To the best of the CGC’s knowledge, this is the only design tool available in Canada that produces a BIN table based on accurate municipal meteorological data as well as other design parameters not considered by other software. This tool is more accurate for residential as it considers the overall performance based on all components, not only the rated performance of a heat pump unit. When used in conjunction with C448 requirements, the GeoAnalyser provides a much better estimation of the 70 per cent / 90 per cent design requirement and also produces a more credible SCOP.

Benjamin Hénault is a professional engineer and a graduate of École de technologie suprieure. He is the technical advisor at the Canadian GeoExchange Coalition and is studying in geothermal research for his masters degree at École Polytechnique de Montreal.