Professor Hussam Jouhara                            OSE0002

Written evidence submitted by Professor Hussam Jouhara (Head of the Heat Pipe and Thermal Management Research Group, Brunel University London) [[1]]

Executive Summary

• The efficiency of a solar panel is adversely affected by increased temperature. They are tested at 25ºC, but ironically, exposure to sunlight increases their temperature and so reduces their efficiency, typically lowering it by 0.26% for every degree Celsius above 25ºC.

• This evidence is submitted on the understanding that there are several variations on the methods of cooling photovoltaic cells, including those that utilise the recovered heat for water heating. What this submission draws to the committee’s attention is the integration of flat heat-pipe technologies into a complete energy-active roofing envelope material – thereby doing away with the need for other more traditional methods of roof cladding.

• This hybrid technology then being used to pre-heat domestic hot water for radiators, baths, and showers – meeting 60% of a dwelling's hot water needs for days with low levels of solar radiation and 100% on days with high solar radiation – whilst (at the same time) also generating electricity.
   
• This submission therefore principally addresses the question: What role can developments in solar panel technology play in the UK’s transition to net zero?

• Plus, to a very limited extent, the questions:
  (i) What are the current barriers (regulatory, technological or otherwise) to expanding the number of small and large- scale solar installations in the UK?
  (ii) Are there opportunities for solar energy generated abroad (e.g., in the Sahara Desert) to be delivered to the UK via interconnectors?

1. What role can developments in solar panel technology play in the UK’s transition to net zero?

1.1              The technology

1.1.1        One of the enigmas of photovoltaic (PV) cells in converting the energy of sunlight directly into electricity is that the greater the level of sunlight the higher (in theory) the electrical output – but often in reality, the greater the level of sunlight, the higher the cell temperature with a corresponding reduction in conversion efficiency somewhere between 10 – 25% depending on cell type and location [[2]].
 

1.1.2        What is suggested to the committee is consideration of a heat-pipe solar roof which combines flat heat pipes with PV cells – to both heat water and generate electricity – but which are also fabricated in the form of weather-tight roofing panels which simply click together to form the finished roof [[3]].

1.2              What is a heat pipe?
A heat pipe is – as the name suggests – a heat-transfer device, but one which contains a liquid which picks up heat from one end of the ‘pipe’ and in doing so boils and changes into a vapour. This vapour then quickly travels to the other end of the heat pipe where it gives up this heat and condenses back into a liquid – which then returns to the hot interface to repeat the cycle. The benefits being, that due to the very high heat transfer properties of boiling and condensation processes, heat pipes are fast and highly effective thermal conductors. The effective heat transfer rates depend on the length and construction of the heat pipe, but typically they can reach heat transfer rates 250 times higher than say just a simple copper pipe or heat sink [[4]].

1.3              Flat heat pipe solar roof
To facilitate the required action and still maintain a low profile, Professor Jouhara and his Brunel research team, designed a flat (low profile – only 4mm thickness) heat pipe (which they term a ‘heat mat’) to optimise the collection of solar radiation. The research and development were undertaken through a DECC (Department for Energy and Climate Change) and Royal Dutch Shell funded project, collaborating with Econotherm (UK) Ltd., Flint Engineering Ltd., and Enertek International Limited [[5], [6]].

1.3.1        Simply adding solar panels to a convention roof and insulating the house below is not a good solution as that simply traps heat, driving up the PV panel temperature and further lowering its performance. With this hybrid system there is no waste heat.
 

1.3.2        A roof solar thermal heating system based on flat heat pipe modules extending the height of the sloped roof was manufactured [[7]], installed in a test UK three-bedroom detached house trial building at the Building Research Establishment (BRE) in Watford, and underwent initial and further extensive trials.
 

1.3.3        To ensure a fair comparison of the available technologies, five solar-thermal panel configurations were implemented with a water cooling cycle (taking the solar thermal heated water to a hot water storage tank). Two were configured with monocrystalline silicon modules mounted on the heat pipe solar thermal units, the other two with polycrystalline silicone modules, where one of the collectors used the heat pipe solar thermal collector technology alone (no PV) and was equipped with a cooling system, while the last collector did not include any cooling cycle. The duration of the initial experiments was four days during September 2014, and these were conducted under different solar irradiation conditions.

 

1.3.4        These tests were then coupled with simulation results for five of the solar-thermal panels connected with a cooling water tank (volume of 500 L), a domestic hot water tank (volume 350 L) and a water-to-water heat pump, so covering the hot water demands of a single family dwelling. The results showed that the patented [[8]] hybrid solar collectors would be able to cover approximately 60% of the dwelling's hot water needs for days with low levels of solar radiation, while for days with high solar irradiation they could cover 100% of the hot water requirements of the family –This unique mixture of technologies being used to pre-heat domestic hot water for radiators, baths, and showers whilst (at the same time) also generating electricity [[9]].

1.4              Impact of the technology
Water heating accounts for 26% of the total energy consumed in UK residential homes [[10]] and the use of such could well be an adjoint as a source of thermal energy for a heat pump – something again the government is keen to see promoted as a replacement for conventional gas-fired boilers.

1.4.1        Traditionally, the outer skin of a building is used to protect the inside from the environment. With this PV/T (photovoltaic/thermal) solar electricity and solar heating system implemented on the outer skin of the building and integrated into the roof, the building becomes an energy-active element of the heating and cooling needs of the building. The flat heat pipes used measured 4mm (0.4cm) thick by 400mm (40cm) wide and 4 metres long. These dimensions optimise the collection of solar radiation and allow the panels to be integrated between the roof struts. The heat-pipes heat water for use in the home as well as transferring heat away from the solar cells, which means their efficiency is not degraded as much as when simply mounted on the roof. It was determined that the pipes helped the PV cells to cool by 15 percent more than with a standard installation. The conversion efficiency of current conventional solar roof PV panels is up to 20%, while the system in the project could provide energy to buildings with a full (PV plus solar thermal) efficiency of 64% [[11]].

2. What are the current barriers (regulatory, technological or otherwise) to expanding the number of small and large- scale solar installations in the UK?

2.1              In addition to all the other benefits, the costs of the hybrid solar-roof system are only marginally higher than standard roofing, so there are significant benefits in integrating these into new builds [[12]]. I would therefore strongly recommend the principle of supporting and encouraging the use of this hybrid technology by offering financial grants or other such incentives, rather than limiting them to ECO4 (Energy Company Obligation), LA Flex (Local Authority Flex), or the Smart Export Guarantee (SEG) Feed-in-tariffs [[13]].

3. Are there opportunities for solar energy generated abroad (e.g., in the Sahara Desert) to be delivered to the UK via interconnectors?
    

3.1              The generation of solar power in locations such as the Sahara Desert would have a vastly degraded performance unless some form of cooling of the PV cells were implemented. As such, the heat mat design would prove very beneficial, flexible… and very necessary [[14]].

November 2022