In 2013 I wrote a book — Renewables: A Review of Sustainable Energy Supply Options – that was published by the IOP Publishing, which publishes Physics World. The book examined the different types of renewable energy solutions and for each one accessed progress, problems, impacts and opportunities. Yet so much has changed in the past six years, that the book is now out of date. So, for the past year I have been working on a second edition — a major update and extension, which I am pleased to say is now available to read.

As for the first edition, the book assesses the current state of play in renewables. It begins by looking at forms of renewable energy – such as hydro projects, wind, wave- and tidal-driven devices — in which natural energy flows are tapped and converted into mechanical power and then electrical power. For these types, it hasn’t all been plain sailing. Hydro, which has so far seen 1.2 TW of installed capacity, has struggled with negative environmental assessments and in some cases unreliable rainwater supply due to climate change.

There have been huge breakthroughs in solar photovoltaics, which is now heading for 500 GW globally

Smaller projects, including “run-of-the-river” schemes without reservoirs, are often favoured by environmentalists, but projects with large reservoirs can play an important ‘pumped storage’ role in grid balancing. The same may be true of large tidal barrages and lagoons, but the former are usually opposed by environmentalists as being too invasive. Tidal current turbines have proved to be far more popular and also easier to develop than wave-energy devices, but many projects of all types are under development. We might expect gigawatts soon and eventually terrawatts.

However, Wind power, both on- and off-shore, has been the big new technology success. As costs fall dramatically and capacity heads for 600 GW, 5-10 TW or more may be possible longer term, especially if airborne devices work well. Large floating devices are a breakthrough technology that can operate in deep water far out to sea, and like offshore wind in general, avoid the land-use and visual intrusion issues that have sometimes constrained on-shore wind projects.

Hot topic

The book then looks at systems — like biomass, solar thermal, Concentrated Solar Power (CSP), geothermal – that use natural sources of heat either directly or to generate electrical energy. Biomass has faced even tougher land use and eco-impact constraints and the new book goes through the sometimes rather tortured debates over the impact of the use of forest-derived biomass on carbon balances and carbon sinks, and the impacts of vehicle biofuel production. Views clearly differ on whether biomass can be relied on as a major source of heat, power and transport fuel, but some look to the use of bio-wastes to avoid the land use problem.

A simple view is that energy storage will solve everything — especially as it’s getting cheaper. Sadly, it’s more complex than that.

Direct solar heat use has had far fewer problems. It is heading for 500 GWth globally, with heat stores offering a way to use summer solar heat in the winter — but at a price. Focused-solar CSP conversion to power has a large potential but has been less successful so far with only 5 GW installed capacity globally. Yet CSP does have a heat storage option, so that it can be run continuously. Other techniques such as solar chimneys, solar ponds and ocean thermal solar devices are also reviewed. They all hold some promise but are very location specific. Geothermal is also making progress (13 GWe, 28 GWth, so far) and the long-term global power potential is large at 2-3 TWs or more.

Rapid enrollment

Finally, there are devices, such as solar photovoltaics (PV solar), which convert solar energy directly into electricity. There have been huge breakthroughs in this area. PV solar is now heading for 500 GW globally, with costs falling very rapidly, so much so that multi-TW deployment is planned in the years ahead, maybe as much as 20 TW or more by 2050. Some of this is due to new technology – more efficient high-tech, multi-junction cells, some reaching up to 40% more conversion efficiency.

However, what has really changed is the advent of cheap, easier to mass-produce thin-film or dye-based cells, increasingly using non-toxic materials. They may have lower efficiencies, but they can be rolled out for many new applications – for example, for solar windows. PV is also being used for solar roads, solar carport canopies and now increasingly floating arrays, for example on reservoirs.

This helps to deal with one of the big drawbacks of PV — it takes up space. Unless, of course, you want to actually put PV arrays in space. That deals with the other big drawback of using solar — it gets dark on one side of the planet at night. However, there are cheaper options than launching PV into space and microwaving power back, with new storage options now emerging, along with new long-distance supergrid possibilities.

Balancing power

Renewables now supply over 26% of global electricity and another key topic that the book discusses are integration issues, including grid balancing, transmission and storage, followed by a roundup of global progress. Grid balancing and integration is an area that is expanding in importance but is complicated. A simple view is that energy storage will solve everything — especially as it’s getting cheaper. Sadly, it’s more complex than that. Batteries are getting cheaper, but they can only realistically store power for a short time — a few hours or days at most. They can also be used to deal with short-term voltage and frequency perturbations on the grid. But for longer-term variations and long lulls in power availability, you need large bulk storage systems. Pumped hydro can perhaps cope for a few days if it has large reservoirs. For longer than that you need something extra. Options include compressed air and hydrogen gas stored in vast underground salt caverns. Energy top-ups could also be obtained from overseas using supergrid links.

This new supply and storage system would be complimented with a new management system that is able to shift demand peaks to times when more power was available — for example, by variable pricing so that power costs more at peak times. Optimising it all will be hard it should be possible to move towards a balanced sustainable energy system.

The second edition of the book ends by taking on some anti-renewable contrarian viewpoints. It also explores the scale issue. Some say we should stick to small, local-scale technology. No spoilers here, but the above should indicate that, although local projects can and should play a major role, they may not be enough — larger systems are also needed. Also maybe it will be no surprise that nuclear is not seen as playing a major role, or fossil fuel carbon capture. Instead, like the first edition, the book tries to present a coherent case for renewables, which six years on, seems stronger than ever.