Would Photovoltaics Cut the Mustard
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.
Holdfast Training Services Trial Blog 
The guys on my design team suggested a handy tool for getting quick PV calc results when trying different values – its the European Commission’s “PV Geo Info System” web page;
https://re.jrc.ec.europa.eu/pvg_tools/en/tools.html#PVP
If you are happy with the assumption that 7.5m^2 of panel will give you 1 kWp, you can play with the values to figure how much roof-space you would need (I’d provide an additional 30% of roof-space space for access (to install and maintain).
After you enter your location, it will give you optimum tilt/azimuth, or you can enter manually.
It also breaks-down the annual output per month so you can see what you are generating with seasonal variations.
Giving it a blast; I recon at tilt/azimuth 33/180 you’re after 1350m^2 of roof space (1755 with +30%). If you could optimize the tilt/az to 40/-4, you’d only need 55% of the area in theory, although you might then require more space to prevent self-shading with the new tilt?
If you start looking into battery storage, I’d recommend jumping on IHS and downloading BRE’s “Batteries with solar power” (2016).
So I used the National Renewable Energy Laboratory software ‘System Advisor Model’ and integrated the European Commission PGVIS weather data with that for London to estimate power generation. I validated the PGVIS data for london with CIBSE Guide A values which was reassuring. That’s where my values came from, but very good point about the tilt/azimuth optimisation, there was an option to model the system with two axis tracking within azimuth and tilt boundaries which boosted annual energy production by about 30% for my model. Maybe give the software a whirl, it’s extremely straightforward. Cheers for the pointer reference batteries.
Given where you are working John, listed building consent is also not going to be a small problem for a highly visible installation. Has this aspect of the work been considered in conjunction with Historic England?
As far as i’m aware this aspect hasn’t been considered. The sustainability ‘strategy’ is still very much in it’s infancy, we are in the process of preparing tender documentation for an external specialist consultant. From what I can see the emphasis from the sustainability working group has so far been placed on introducing renewable energy systems and not necessarily appreciating the importance of a ‘whole building approach’ as outlined in the Historic England document ‘Historic England, Energy Efficiency and Historic Buidings’ which provides a great insight into the complexities involved with renovating listed buildings. Or in other words, the emphasis has been on the ‘Green’ elements rather than the fundamental ‘Mean’ and ‘Lean’ aspects of sustainability. CIBSE Journal has a specific article on the sustainable renovation of victorian buildings within the Westborough Academy which represents the gold standard for a project that synergised all methods of improving energy efficiency and not just one niche area.
I firmly believe that there may be alternative options to a heat pump coupled with PV for the swimming pool that offer better value for money in terms of carbon emission reduction. It is the principal designer that suggested the heat pump, there’s the possibility they’re focused more on the reservicing works than delivering a cutting edge, innovative sustainability solution. For example the wireless thermostatic radiator valves which have recently been ommitted may have provided much better carbon savings for the investment than any number of renewable technologies, it’s just that no one has looked at it in detail. Secondary glazing is also an extremely promising building fabric improvement, Historic England have commissioned studies into this showing the potential for dramatic reduction in heat loss, clearly this would need to be considered against the moisture and condensation control within the building but that’s where a specialist consultant comes in.
The golden rule of carbon reduction investment is reduce your existing losses to a minimum before you start looking at fancy new technology.
My sentiments exactly.