Photovoltaics: the state of the industry and routes to high efficiency

We have re-started our weekly, daytime, energy seminars for the spring term. Today we had Dr Nic Hylton a new member of staff at Energy Futures Lab who comes to us from the Department of Physics here at Imperial College London. His talk was on deployment of solar PV and its possible futures. As usual we have a complementary blog post if you missed the talk or want to go over his talk again.

Deployment of solar PV installations is on the rise. Fuelled by plummeting costs and policy schemes designed to make renewable energy sources more attractive, annual installed capacity exceeded 50 GW in 2015. According to the report from Solar Power Europe on the Global Market Outlook, this took the total cumulative capacity to around 230 GW. Digging a bit deeper, a breakdown of global PV deployment by region shows that installations in Europe led the charge around the turn of the decade.

Solar panels

Since 2011 however the annual deployment in Europe has dropped off, but this reduction in newly installed capacity has been more than compensated for by growth in Asia and America, leading to a continuing rise in capacity to date. The International Renewable Energy Agency, IRENA, produced a report in 2016 in which they claim that the primary driver for the solar revolution has been a dramatic reduction in price. In 2015 the global average levelised cost of electricity from PV was just $0.13 /kWh they say (in the UK this figure was $0.093 /kWh). This compares reasonably well with the price of electricity from conventional sources – around $0.05-0.10 /kWh – and IRENA predict that with the right support from enabling policies the price of solar PV could drop by as much as 59 % by 2025.

This is all good news for the global PV market, but if further price drops are predicted to be policy dependent then what is in store for the UK? In late 2015 the British government announced dramatic reductions in the value of the feed in tariff for PV installations. Clearly this is a challenging time for UK solar. A more gradual reduction in the feed in tariffs in the move towards zero subsidy would surely have been preferable, but the UK solar industry has shown encouraging innovation and development and in conjunction with storage technologies can be self-sustaining.

High efficiency PV research

Although these recent reports have shown that PV costs are dropping and despite the fact that the majority of new PV installations employ single junction silicon solar cells, there remains considerable motivation in the research community to push towards new PV technologies and higher efficiency.

Intermediate band ‘quantum ratchet’ devices under the microscope
Intermediate band ‘quantum ratchet’ devices under the microscope

The research team that forms the Quantum Photovoltaics (QPV) group in the Physics department at Imperial College London work on a variety of projects targeting high conversion efficiency. My research while a member of this team covered a range of topics from fundamental research in novel concepts like photon upconversion and intermediate band solar cells, through to materials development on new semiconductor alloys for the next generation of multi-junction devices.

In an upconversion material the idea is to capture low energy solar photons that are not normally absorbed and convert them to higher energy photons that can be usefully harvested by the solar cell. Similarly, in an intermediate band PV structure, the aim is to create new energy states in the solar cell that enable the absorption of low energy photons that would otherwise be lost. Progress is being made in both of these areas by researchers in the QPV team and other groups around the world.

Meanwhile, development of new semiconductor alloys for the next generation of multi-junction solar cells continues apace and the world record efficiency continues to creep upwards. Despite the harsh cuts to subsidies recently imposed by the government the outlook for solar PV is bright.

Spectroscopy of solar cell materials in the laser lab
Spectroscopy of solar cell materials in the laser lab

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