Sunshine wins: PV solar progress

As we struggle with Covid 19, while still needing to deal with climate change, I thought I would look at some uplifting success stories in the renewable energy area. Solar energy offers just that.  Its uptake has been driven by rapidly reducing costs, as volume production increased and technology improved. For example, back in 2017, solar auctions in Mexico yielded an unheard-of average price of $20.57/MWh, including a $17.7 bid by Enel, this beating an earlier $17.9/MWh tender for a 300 MW PV plant in Saudi Arabia. Some of these prices may be exceptional, unsustainable  and locally specific (in sunny countries), but clearly prices were falling around the world.  Lazarad’s 2018 global review put the Levelised Cost of Energy for utility scale PV, without subsidy, at $44/MWh, falling to $32-42/MWh for thin film systems in its 2019 review.

New materials

Thin film cells, with the cell material deposited in a very thin layer on a substrate backing, are the cheapest so far, but thin film ‘amorphous’ silicon cells have lower efficiencies (10% or less) than mono or poly crystalline cells (15% or more). So there is a trade off between cost and efficiency. At present, crystalline silicon wafer-based cells have around 90% of the market, but thin film cells are edging in, with new cells using more exotic materials emerging e.g. copper indium selenide (CIS), copper indium gallium selenide (CIGS) and cadmium telluride (CdTe), these sometimes being called ‘second generation’ cells, some of them edging up to 20% efficiency. CdTe cells have proven the most popular, taking around half the market for thin film cells. 

There are also some other options. Gallium nitride (GAN) cells have been seen as promising, while gallium arsenide (GsAs) cells, although expensive, can have efficiencies of up to 28%. CZTS thin film cells, made up of Kesterite, Cu₂ZnSnS₄, offer optical and electronic properties similar to CIGS, but unlike CIGS (or other thin films such as CdTe), are composed of abundant and non-toxic elements. The later point is important: once installed, PV has no emissions, but the use of toxic materials presents health and safety issues during manufacture and care must be taken with eventual cell disposal if they contain toxic materials. As with all electrical and electronic systems there is also a risk of fire, and risks to firefighters from possible toxic emissions.

While the search goes on for new materials, the likely best bets in efficiency terms are multi-layered multi-junction versions of these various cells, with combinations of different materials in tandem. That can push cell efficiency up to around 30% in some cases, since each layer consists of material chosen to absorb light of a different wavelength. However, getting much past 32% without light focusing/light concentrating devices seems to be hard. But with light focusing, some devices have got above 40%. One of the highest overall PV efficiency achieved so far is 44% at ‘947 suns’ concentration, with a multi-junction concentrating solar cell.

Solar farms

The attraction of light focused ‘Concentrating PV’ (CPV) systems, using lenses or parabolic dish mirrors, is that they can use small areas of expensive high efficiency solar cell- mirrors or lenses are cheaper than cells!  But, for focused solar, you have to have direct, rather than diffuse, sunlight, so CPV projects are usually deployed in desert areas and so far only around 350 MW of capacity has been built.  By contrasts there are many more large conventional PV arrays in desert arrays, with several initially in the USA and projects now over 1GW in China and India. Indeed, India is planning one at 5GW for 2023.

There are also many smaller arrays elsewhere around the world, including many solar farms in the EU. The spread of large solar farms has led to some land-use issues. Clearly PV on the roof-tops of houses, offices, factories and warehouses and so on takes no extra land, and that type of location is to be preferred over PV arrays on the ground, even if the latter are usually easier to deploy and more economically viable. However, there are limits to how much suitable roof top or wall space is available, and ground-mounted PV seems bound to spread.  In some high population density countries, where open land is scarce and farm land very valuable, this has led to some planning conflicts, with controls being imposed, as for example in the UK. There had certainly been some local complaints about visual intrusion, but there were also dissenting views about the likely environmental impacts of solar farms in rural areas. It was argued that they could still allow for sheep grazing and create wild flower and wild life havens. An intriguing idea to avoid the use of agricultural land is solar roads, with armoured PV materials embedded in road surfaces. However, although some have been built in France and China, it has been argued that the cost/km is too high and the solar gain too low.

Solar on lakes- and in the sky  

A more radical way to avoid land-use issues is to opt for floating solar arrays on lakes and reservoirs, as was pioneered in, amongst other places, the USA, UK, and Japan.  Floating arrays also have the advantage of reducing evaporation and also providing cooling for the PV cells. That has been especially relevant for the projects emerging in India, China, and Brazil.  These are all relatively small schemes, but larger ones are on the way: see my next post.

Putting solar cells in orbit  in space and micro-wave beaming power back to earth has sometimes been discussed and I will also look at that in my next post - but for the foreseeable future projects are likely to have to stay on earth. There, some of the large arrays may not only have to face land-use issues, but also issues relating to the water needed for cell cleaning- sand storms in desert areas can leave a significant problem and in urban environments grit and dirt can be deposited over time on the cells, reducing their performance. So, self-cleaning surfaces are important to reduce the need for water and detergents.

In desert temperatures especially, some cooling will also be needed, given that PV cell efficiency falls with increased temperature. Moreover, climate change may lead to periods of higher temperatures in many places, not just in deserts. Some developers have produced hybrid solar thermal/PV systems, with, for example, a semi-transparent PV sheet on top of a heat absorbing solar collector.  This ‘PV/T’ approach can not only keep the PV system cool, so upgrading its efficiency, but also doubles up on land, roof, or wall space usage. Otherwise you can have conflicts between space for PV cells and space for solar heat collectors. PVT can be highly efficient. A PVT system has been developed in the UK with PV cells in an evacuated tube heat collector for roof-top use, with its developer claiming that its electrical output is 20% higher than comparative non-PVT units.

What next?

Solar PV has come a long way since the early days when fabrication costs were high, partly due to the high energy cost of cell manufacture. Now energy (and therefore carbon) pay back times are put at between 1 and 2.5 years depending on location and cell type. However, the search for improved cell materials continues, with many ideas being developed for so-called third generation solar cells, with lower costs and easier fabrication. That category includes various types of ink dye-based cells, organic/plastic (polymer) cells, graphene cells, perovskite cells and quantum dot cells- moving away from silicon and the more exotic materials. Perovskites do seem to be gaining ground and getting to higher efficiencies  

Some ink dye cells, although of lower efficiency, can be very low cost, and some can be printed or even spayed on to flexible surfaces. One application is for solar windows, using a translucent layer of material. Some see that as a transformative development.

Roof top mounted domestic PV and large scale PV arrays are already booming, so it is perhaps not surprising that, with new techs and applications also emerging, including thermo solar options, the prospects for solar have been talked up. For example, the DNV-GL consultancy’s Energy Transitions Outlook looks to solar power having a 40% share of total global electricity generation by 2050, while Shell’s ‘Sky’ scenario has solar supplying 32% of total global energy by 2070. Topping all of these, LUT/EWRs latest scenario has PV supplying 69% of global primary energy by 2050. So although the economic constraints following on from Covid 19 may slow things in the short term, solar could be going places longer term - although one big question is, will there be enough room for it?  See my next post.