Hybrid Low Carbon Hot Water Systems For Commercial Buildings

Achieving 60°C in a calorifier is a basic requirement for a commercial domestic hot water (DHW) system. But achieving this through a direct electric-only immersion system is costly, even when compared to current surging gas prices. For this reason, air source heat pumps (ASHPs) would appear to be a perfect alternative, supplying low carbon heat to the building. However, with ASHPs offering greater efficiencies in low-temperature systems, the high-temperature demands of commercial applications prove challenging. High working flow temperature from the ASHP would need to be at least 65°C, but to achieve this requires greater compression of the refrigerant, requiring more electrical input which results in lower efficiency or Coefficient of Performance (COP). As well as struggling to achieve these temperatures year-round in the UK, the very low drop in COP counteracts the value from deploying ASHP.

A working flow of 55°C is however attainable all year round from an ASHP. If employed as a source for a preheat, the heat pump can be used to offset the direct electric costs of top-up after heat. Therefore, commercial new builds, where gas is less likely to be specified, are defaulting to this hybrid option.

Nonetheless, there remain a series of challenges. Aside from the additional system complexity, when connected to a traditional indirect water heater the lower flow temperatures generated by an ASHP lead to around a 50% drop in energy transferred. This is because ASHPs will typically have 5 K temperature differential (55°C flow vs 50°C stored) compared to 20 K Delta (80°C flow vs 60°C stored) of gas-fired boilers. The size of the coil in the indirect calorifier can additionally limit heat transfer and affect system capacity. But for all-electric hybrid systems by far the most common problem comes from how the calorifier’s coil and the immersion interact.

An efficient electric hybrid approach depends on harmoniously balancing these different system elements to ensure that they do not work against each other. If the coil and immersion are too closely situated, they become impossible to accurately control resulting in a steep drop in system efficiency.

To avoid these issues, a hybrid system would, up until now, be best served by employing a plate heat exchanger (PHE) with low-temperature hot water (LTHW) and domestic hot water (DHW) buffers alongside the ASHP. That of course further exacerbates the system complexity, space requirements and capital costs. For smaller to mid-scale hot water applications those capital costs can be hard to justify, forcing them, if possible, to be written off against corporate sustainability gains.

In response, Adveco has been working in close cooperation with its customers to address the core challenges of delivering hybrid all-electric hot water heating into commercial building projects using ASHP technology. Evolving from bespoke system builds to award-winning packaged plant room applications, it became apparent that there is a growing demand for lower carbon pre-sized systems that are easy to specify, straightforward to install, resilient and cost-effective. This evolutionary work has led to the creation of Adveco’s FUSION FPH-S range of all-electric, packaged hybrid hot water systems.

FUSION harnesses the highly respected FPi32 ASHP, a high-pressure ATSH calorifier with electric immersion, controls, and metering. This provides a reliable, high-temperature, sustainable and cost-effective system for meeting continuous hot water demands (from 257 up to 377 litres/hour) found in new commercial builds as well as refurbishment projects where gas and associated flueing is either not possible or no longer desired.

The system specification is available in 16 variants with 6 or 10 kW preheat and 9 or 12 kW electric top-up.  Capacities ranging from 200 to 500 litres all rated at 10 BAR for high-pressure applications. This makes FUSION highly adaptable for a wide range of commercial applications (offices, small schools, retail sites, cafes and restaurants). The FPi32 ASHP is specified to supply the preheated hot water up to 55°C throughout the year, even when ambient air temperatures drop as low as -25°C. The stainless steel construction of the ATSH also makes it an excellent all-rounder, resistant to soft water corrosion and, with Adveco’s own low heat intensity electric immersion specification (6W/cm2) the calorifier is more resistant to scale build-up in hard water areas.

FUSION ensures the ASHP preheat and immersion work seamlessly to deliver the highest operational efficiencies. Physical design spaces apart the low-situated high-efficiency preheat coil from the electric top-up immersion. Then dedicated controls and integrated metering monitor temperature and water flow throughout the system. This enables FUSION to make the greatest gains possible from the heat pump even though ambient temperature and system demands will fluctuate. These gains are then used to offset as much direct electrical top-up as possible, reducing carbon intensity and helping control the operational costs of providing business-critical hot water. And without the need for a PHE or additional buffer tank, the capital costs of a hybrid system become more manageable while making major gains in terms of space-saving which is identified as a core requirement for smaller properties that may lack dedicated plant room space.

For organisations with reasonable (non-bathing) daily hot water demands and a desire to embrace a more sustainable business model, FUSION provides a single, easy to accommodate, highly effective response. One that provides optimum efficiencies, lowers carbon and assures building regulations are being met on commercial projects.



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