Close up of the Sun

Space-based solar power: the new space race

Today's agenda for space is no longer focused on merely getting there. The modern space race is about getting the engineering in place to exploit space-based solar power (SBSP). But how will energy be beamed back down to Earth without breaking the bank?

As the UK increasingly relies on energy imports, a sustainable and renewable energy solution needs to be found sooner rather than later. In Britain we are facing the distinct possibility of power cuts this winter, following Ofgem's announcement that the margin of spare capacity could be as low as 5 per cent if we have a particularly cold winter. One solution to our increasing power needs is to create renewable energy in space. The idea is not new. But are we anywhere near generating endless power in space?

Why space-based solar power hasn't happened yet is, as with most things in this world, because we don't know how to fund it. The biggest reason that we haven't yet tapped into this carbon-emission-free, renewable energy source is money. The cost of building, launching, maintaining and harnessing SBSP has been prohibitive. If you care more about money than you do about environmental sustainability, that is.

Whatever your environmental views, there's no getting away from the fact that it will cost hundreds of millions of pounds to get an SBSP programme up and running and powering the UK. But it also costs millions of pounds to build nuclear plants and to implement fracking programmes. So, ultimately, it comes down to priorities. We have to face up to the fact that we need a viable renewable solution if the younger generations, and those yet to be born, are to have reliable energy sources and a habitable planet in the future.

Engineer Ian Cash, who has decades of experience providing electronics design consultancy services to the automotive, aerospace, defence and power industries, thinks he may have come up with that viable solution, namely the HESPeruS (Highly Elliptical Solar Power Satellite). Meanwhile, the Japanese think they've come up with a great solution too. 

Cash currently has a paper lodged with the British Interplanetary Society (BIS) for his HESPeruS design, which he believes has cost parity with new nuclear and is a real contender for solving energy sustainability problems. HESPeruS uses a highly elliptical 12-hour orbit (Molniya), which travels out to 40,000km over the northern hemisphere (450km in the southern). It takes less propellant to reach Molniya than GEO and historically costs half as much.

The Japan Aerospace Exploration Agency (JAEA) is currently working on several models for solar-collecting satellites, which would fly in geosynchronous (or geostationary – GEO) orbit 36,000km above the receiving stations. In the most basic model, the photovoltaic-topped panel's efficiency would go down as the Earth turned away from the Sun but the advanced model has two mirrors that reflect sunlight onto two photovoltaic panels. While the advanced model would be more difficult to build, it has the advantage of generating electricity 24-hours a day.

Commenting on the developments at JAEA, Cash says: "The simpler rigid design that the Japanese are proposing loses power as it turns away from the Sun. To get constant power delivery from GEO 24-hours a day requires a more complex 3D design with rotating parts. The free-flying mirrors would need continual use of propellants to maintain relative position to the photovoltaic panels and microwave generator, or the alternative is to build multi-kilometre long trusses with rotating joints for the reflectors."

In contrast, Cash's HESPeruS design is a rigid, modular and essentially flat design that rotates just once per-year to keep its panels permanently facing the Sun.

Beaming the power down

It's not just in the design for catching the Sun's rays that HESPeruS appears to offer a better solution either, it's also coming out ahead in the way that the power generated reaches Earth.

In JEAE's designs both microwave and laser power beaming are proposed, with only microwaves able to deliver the power through cloud cover. Cash says: "A small transmit antenna, less than 1km say, simply cannot focus a microwave beam over 36,000km to a conceivably-sized rectenna farm. HESPeruS uses the same 5.8GHz ISM band and retrodirective to pinpoint the rectenna."

This is where Cash believes the HESPeruS has the biggest advantage over other designs: "It's perhaps unique in having a transmitter diameter (aperture) greater than its solar collecting surface, meaning that at 10 per cent scale HESPeruS could still deliver 100MW of power to the grid while operating in the Molniya orbit."

But not everyone is of the mind that a Molniya orbit can do what is needed. Many argue that GEO is still the only orbit that can deliver 24-hour power. Cash acknowledges that a single HESPeruS satellite cannot deliver power to the UK when it is in the southern hemisphere. "However, HESPeruS is capable of 75 per cent use – meaning four one-third-scale (330MW) satellites can be placed in time offset orbits so that three of them will be delivering a total of 1GW to the same location (three adjacent one-third-scale rectennas)."

Another argument that rages is that a satellite in a Molniya orbit can only deliver power to Arctic (or Antarctic) regions, whereas GEO covers a much greater surface area that contains the majority of the world's population. However, Cash cites a Wall Street Journal article written by Lawrence C Smith in 2010 which revealed that north of the 45° N parallel – where you find the US, Canada, Greenland, Iceland, parts of Japan, the UK, Norway, Sweden, Finland and Russia – there is a not to be sniffed at 15 per cent of the planet's surface area and just under a third of its ice-free land.

Skylon success

It would seem that Cash's design has addressed all of the issues that have hindered the development of SBSP in the past. But there is one thing that this concept relies on that is not yet a reality, and one that is out of his control – the Skylon space plane. The concept design from Reaction Engines for the Skylon space plane uses SABRE, a combined-cycle, air-breathing rocket propulsion system, which means that it could potentially be reusable for 200 flights, compared to the one flight that current spacecraft make. This is because the SABRE technology enables the space plane to take off and land on a runway, just like planes in the aviation industry.

Despite all of the advances that Cash has made in his design, without the success of the Skylon project there can be no HESPeruS. If they become a reality, the Skylon space planes would be used to make the regular construction visits needed to grow HESPeruS to full size (1GW) in under two years. A fleet of 11 will be needed to achieve this goal.

However, as Reaction Engines have been working with the government and other space plane developers on plans for a new spaceport and developing a regulatory framework for space tourism, it would seem that progress on the project is good. When asked about a plan B, in case the Skylon project doesn't deliver what everyone expects it to, Cash doesn't believe one is needed. "All of the information coming out of the project to date is pointing in the direction of success," he says.

Impressive power generation

If Skylon goes ahead and HESPeruS moves beyond concept design to reality, what can we expect from it in terms of power generation? Well, a HESPeruS rectenna site delivering 1GW to the grid would deliver 24GW-hours of energy per day.

Currently, the daily energy requirement per person in the UK is 125kWh. To deliver that using power from HESPeruS alone would require just over 300 rectenna sites to be built. These could be located on land, floated at sea, or strung between offshore wind turbines. Cash, however, sees the future of power generation being a combination of SBSP and other renewables.

How does SBSP compare to the renewable energy sources that are already a reality rather than a concept? The power density of a solar PV farm is approximately 10W per square metre. If 5 per cent of the UK land mass not already used for building, pasture, forestry or agriculture were to be covered with PV farms, which equates to just over 1,200 square kilometres of land, and added an additional 10 square metres of rooftop PV per person, then solar power could conceivably supply 44 per cent of a person's daily energy requirement. Cash states that for this to become a reality there is the additional proviso that the grid-scale energy storage issue is solved.

Offshore wind generates approximately 3W per square metre, while onshore wind is slightly lower at 2W per square metre. If we built the conceivable limit of wind farms on UK land and in our territorial waters, the two combined could supply 54 per cent of a person's requirement.

However, it would take a considerable amount of time to get enough solar and wind farms built and generating this amount of power. And our power needs will increase. The UK population is predicted to grow to 73 million by 2037 according to projections by the Office of National Statistics in 2013.

Whereas in Cash's design, a HESPeruS rectenna with microwave beam limited to 300W per square metre at its centre (ie, one-third of equatorial midday Sun strength, with equivalent one-third heating effect) can capture 84 per cent of the beam within a 4.6km diameter, and deliver 85 per cent of that to the grid. This gives an average power density of 58W per square metre, which equates to six times better than a solar PV farm, 20 times better than offshore wind and 30 times better than onshore wind.

Space-time delivery

Last year, based on successful tests of the SABRE technology, the UK government pledged a further '60m funding for the Skylon project and, if all goes to plan, the first Skylon test flight is expected to happen in 2019.

Once the HESPeruS design has been through the review process, if it's deemed as a viable solution and moves into development, realistically the UK could see homes and businesses being powered by SBSP in around 15-20 years time.

This is where we get back to where we started. Getting anything so 'high concept' off the ground requires not just considerable funding and investment, but political backing. With the current government's focus on fracking and new nuclear development, it's by no means guaranteed that this support will be forthcoming.

However, as Cash points out, when you consider the other renewable options and what they can realistically deliver, it appears that this is a completely viable solution that will keep the lights on in the long-term. 

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