Holdfast Training Services Trial Blog

The sustainability goal for the project I’m currently working on is a 554,000 kgCO₂ (554 carbon tonne) emission reduction per annum, and it is to be achieved by project completion. Due to two late design ommissions of wireless thermostatic radiator valves and large scale photovoltaic arrays it is projected that this carbon target won’t be realised. To offset these ommissions it is suggested that a heat pump be utilised to provide low grade heat to a suitable application: which the design consultant has identified as the on-site swimming pool hot water demand via two existing plate heat exchangers. To amplify the carbon savings of the heat pump it has been suggested that photovoltaics can be used to provide the electrical demand of the heat pump, there are no explicit calculations to support this assertion.

Design consultant calculations:
Between 38 – 58 kW water heating demand based on modelling software.

That is all the design information that the consultant presents, below are some of the questions that I’ve worked my way through to arrive at an overall conclusion.

Small Selection of Heat Pump Specific Questions:
Can the consultant software be relied upon?
Can I validate their hot water demand calculation with an energy balance equation?
What local viable heat sources are there?
What are the seasonal temperature profiles for each source?
What are the theoretical seasonal co-efficient of performance (SCOP) associated with each heat source?
Why use SCOP? Does the heating demand of the pool vary with the season?
What is the real SCOP likely to be based on empirical evidence?
Is the hot water demand of the pool constant? 24/7? all year round?
How do I obtain an electrical energy load profile for the heat pump?
Won’t the electrical demand of the heat pump vary with source temperature profile?
What should I consider to determine worst case operating conditions (Highest electrical demand for the heat pump) i.e highest flow temperature required, coldest heat source temperature, lowest plate heat exchanger efficiency…?

Small Selection of Photovoltaic Specific Questions
What determines the viability of a photovoltaic solution: cost, land required, installation practicalities, servicing requirement?
Is battery storage necessary and if so how will it be designed to modulate the electrical supply and demand between the PV array and heat pump?
How can I best estimate the annual PV energy yield of different sized systems?
How can I validate the PV energy yield software/calculations?
Can I utilise a two-axis tracking system? Would the extra cost be offset by the increased annual energy production?

In a nutshell: Due to space environmental set point temperatures, the swimming pool hot water demand can be approximated as constant all year round with the energy balance equation resulting in a loss of 31.2 kW (Mostly through evaporation and mains temperature renovated feed water). The best heat source is the lake due to the highest SCOP, the COP for the lake can drop as low as 2.6 in winter conditions with a corresponding heat pump electrical requirement of 12 kW. Based on the SCOP (for the highest flow temperature of 65°C, SCOP = 3) the annual electrical energy demand is ≈ 96,000 kWh, with an average electrical requirement of 10.4kW. For a monocrystalline flat panel PV array in London, with a 180° azimuth, 33° tilt to the horizontal, module efficiency of 19% and no shading losses an area of ≈ 500m² is required to provide the average annual electricity requirements of 96,000 kWh. That area requirement exceeds the space offered by the swimming pool roof, and adding into contention the significant shading losses (Due to nearby buildings) and sub-optimal orientation angles from the slope of the roof, it would be necessary to either increase the size of the system or relocate it (introducing the potential for significant voltage drops and increased cable size and cost)

The added complexity of utilising battery storage to modulate and supply the varying electrical input from the PV array to the heat pump would be the next steps to assessing the viability of the coupled system.

In short, the consultants assertion is likely to be impractical.