BEYOND SCOP - ESTIMATING REAL WORLD HEAT PUMP PERFORMANCE WITH SPF
Rinnai Applied Head of Design Sean Ehlen evaluates the importance of heat pump system congruity utilising Seasonal Performance Factors (SPF) as the expected performance. Advanced evaluations and verified methodologies are the preferred modes of measurement. A heat pump’s capabilities should be based on actual operating conditions and locations to yield accurate pictures of on-site performance.
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When investing in a low-carbon heating and hot water system, end users want confidence that the installed solution will deliver the performance, operating costs and carbon savings promised. While metrics such as COP and SCOP are widely used across the heat pump industry, they do not always reflect how a system will perform in real-world operating conditions.
For heat pumps to become effective in mainstream commercial applications a measurement method that harnesses congruity must be utilised, so that expectations on performance can be fully achieved.
There are several ratified methods of measuring the performance of heating and hot water systems more notably heat pumps. Seasonal Coefficient of Performance (SCOP) is regarded as the industry standard of system efficiency measurement. This metric does not however take into consideration several key factors such as location, application, or complete system energy usage.
Therefore, projects can lack congruity, and energy usage can be more than what the initial modelling identified. SCOP is calculated using standardised climate profiles and operating conditions in accordance with recognised European test methodologies. While this provides a valuable benchmark for comparing heat pumps, these assumptions may differ from the operating conditions experienced at a specific commercial hot water installation.
While for commercial hot water applications in the UK, it is preferred to understand the temperature profile of the installation location, e.g., Glasgow, Scotland - and for a water outlet temperature ranging from 55C-65C, depending on the heat pumps capabilities.
SCOP primarily reflects the seasonal performance of the heat pump unit under standardized test conditions and does not fully represent the performance of the complete installed heating and hot water system. It should be recognised that SCOP remains an important and valuable metric for comparing heat pump products under standardised conditions. However, commercial customers ultimately experience the performance of the complete installed system rather than the heat pump in isolation.
Coefficient of Performance (COP) is also regarded as an alternative method of system efficiency translation. This approach is also limited in its scope; COP fails to include system wide losses and instead provides a snapshot of heat pump efficiency at a defined operating condition but does not capture seasonal performance or wider system energy consumption.
Additional areas that are not fully enveloped into overall heating and hot water system considerations are the ignoring of part-load inefficiencies. Heat pump efficiency varies significantly across the operating range, and a single COP value may not fully reflect performance under varying load conditions experienced throughout the year.
Finally, a high COP score does not necessarily guarantee lower operational costs if the heat pump system is not properly sized, poorly maintained or is installed with a too high temperature lift. This occurs when a low-temperature heat pump is used to heat old and small radiators. COP is also renowned for measuring efficiency at only one temperature, meaning that there is a lack of informational scope when compared to SCOP and SPF.
Both a SCOP and SPF theoretical approach are used as more accurate gauges of operational expenditure. However, Seasonal Performance Factor (SPF) is widely regarded as providing a more representative assessment of installed system performance because it can incorporate the wider system energy consumption and operational characteristics that influence actual efficiency and operating costs.
SCOP provides a valuable, standardised measure of heat pump efficiency. However, commercial customers ultimately pay for the performance of the complete installed system. For this reason, Rinnai believes system-level metrics such as SPF provide a more meaningful indication of likely operating costs, carbon performance and customer outcomes.
The key benefits include SPF:
1) Accurate performance measurement based upon the HW application, allowing the user to forecast more precise operational costs.
2) Design a system for the SPF of the customer, allowing customer constraints and requirements to be at the forefront of design. This allows for the best solution for the customer.
3) Reduced carbon production if a customer works with a manufacturers design team to optimize their system from an SPF perspective.
SPF methodologies are supported by recognised industry research, including the SEPEMO project, which established a series of system boundaries ranging from H1 to H4. These boundaries allow varying levels of system energy consumption to be assessed, ranging from the refrigeration cycle alone through to the performance of the complete installed system. This enables a more transparent understanding of how different design decisions influence operational efficiency.
A high SPF rating indicates genuine heat pump system efficiency, which provides more heat than consumed electricity. An SPF-based assessment can provide a more representative indication of likely real-world performance because it considers the interaction between equipment, ancillary energy consumption, site conditions and customer operating requirements. This results in significant cost reduction and system longevity, both of which are attractive to potential customers.
Heat pump component and overall system congruity is vital for customer expectations to be met and satisfied. Avoiding higher outgoing costs over a product lifecycle, increased system sustainability and efficiency are the pivotal areas that appeal to customers that are considering heating and hot water system purchase. SPF targets these areas of system operation to detail a true account of performance.
For heat pumps to become mainstream in commercial applications, performance assessments should consider the behaviour of the complete installed system rather than the heat pump unit alone. COP remains valuable for understanding performance at specific operating points, while SCOP provides an important, standardised measure for comparing seasonal efficiency. However, Rinnai believes that SPF offers a more representative indication of how a complete heating and hot water system is likely to perform in practice, which is why this methodology forms an important part of the company’s heat pump system design approach.
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