By Rikki Stancich in Paris

Like many other solar technologies, the concept behind solar updraft towers, or solar chimneys, has been around for more than one hundred years. Based on the principle that heat rises, the idea is that heat, captured in an enormous greenhouse, funnels upward into a tall, hollow tower. As the hot air moves through the greenhouse and upward into the tower, it propels a series of turbines (similar to Kaplan hydro turbines) positioned around the base of the tower.

Given the correlation between the height of the tower and the diameter of the collector, the height of the tower creates the temperature differentiation to create the airflow that drives the turbines.

The technology relies on radiant heat, rather than direct sunshine, to heat the air inside the greenhouse. This means that, unlike concentrating solar power (CSP) and photovoltaics (PV), the technology can operate in diffused sunlight – including cloudy or wet weather. In good weather, the plant should operate at a capacity factor of 50% plus, according to Australian solar updraft tower technology developer, EnviroMission.

Despite the first prototype, (built in 1982 by Schlaich Bergermann in Manzanares, Spain), having been hailed a success, the technology has yet to be commercially developed. But 2011 may well be the year that the technology proves it weight in gold.

EnviroMission, listed on the Australian securities exchange in 2001, has since been dedicated to refining the technology. Initially, the company planned to build its first project in a North-Eastern rural area of Australia named Buronga, near Mildura. But like its CSP counterpart, Ausra (now Areva Solar), EnviroMission has struggled to get the project off the ground in the absence of adequate policy support frameworks and incentives.

In mid-2009 the company set up its US headquarters in Phoenix, Arizona. At the end of 2009, EnviroMission’s solar updraft tower project was selected out of hundreds of others from  the Southern Californian  Public Power Authority’s RFP response for renewable energy projects.

EnviroMission has since filed land applications in Arizona for two 5,500 acre (2225.85 hectare) sites, suitable in size for development of two 200MW Solar Tower power stations, has negotiated an SCPPA approved PPA for the off-take of energy generated by the first of two planned solar updraft towers, and has appointed international engineering, design and consulting firm, ARUP, as its design engineer.

CSP Today speaks to EnviroMission’s CEO Roger Davey to learn more about the technology and the challenges that lie ahead.

CSP Today: Since the Manzanares pilot there has been little talk of developing solar updraft tower technology, yet now Enviromission has entered the race with a completely new technology and a signed PPA that has gained SCPPA approval. The latest news is that you have engaged Arup for the engineering design of the solar updraft tower. What happens next and when are you likely to break ground on this project?

Roger Davey: In fact a lot was happening –  Negotiations with the SCPPA have been ongoing since November 2009, when it was announced that the SCPPA had selected EnviroMission out of hundreds of applications. We have been working hard ever since, to negotiate the PPA and as of October 21, have it approved, enter into a formal agreement with Arup, land acquisition and all other associated matters. 

The design phase will take around 9 -12 months, which should bring us to the financing stage - to a point at which the project can be banked.

CSP Today: What are the solar updraft tower’s advantages and shortcomings compared to solar thermal technologies?

Roger Davey: The major advantage is that its capacity is a lot higher than other renewables – in other words, solar updraft towers can produce more power per megawatt installed.

Another point that differentiates solar updraft technology from other solar technologies is that it does not use any water in the generation cycle. In the Southwest US where water is a scarce commodity, the technology really fits because it has no water requirement. CSP, on the other hand, uses nearly as much water as nuclear energy.

This gives us a significant advantage over other water dependent technologies, it should be noted  that Arizona has just passed a bill that requires energy sources to be fully costed, including the environmental costs. This includes water, which until now has been a hidden cost.

Another benefit is that we can guarantee the output, unlike PV and wind. Solar updraft technology works (generates electricity) in diffused sunlight and in differentiated temperatures.

So there you have three key points that set solar updraft towers apart from other renewable energy technologies.

CSP Today: How does EnviroMission's technology stack up against CSP technologies from a cost perspective? 

Roger Davey: Well, the SCPPA has just signed a PPA with us, which they would not have done if they weren’t happy with the pricing. So obviously we are competitive.

CSP Today: How does solar updraft technology measure up against nuclear and conventional power generation in terms of cost and output?

Roger Davey: At this stage, it is a difficult question to answer. With regard to nuclear, you have very high costs related to power production, water requirement and decommissioning. Nuclear is probably more expensive.

While coal has a capacity factor in the 80% region, when you start costing in water, [solar updraft technology] is competitive. It should be noted that in comparing a Solar Tower to a similar energy producing coal plant – the Solar Tower will abate approximately 1,000,000 tons of CO2; the equivalent of removing 220,000 motor vehicles off the road and importantly abate the useage of some 528 million gallons of potable water per annum

The updraft tower can produce energy 24 hours a day, seven days a week – but not at full capacity. So it doesn’t have the capacity factor of nuclear or coal.

CSP Today: EnviroMission’s Solar Tower will be the first ever, commercial solar updraft tower. Surely, given that the technology is so new, the initial costs will be relatively high?

Roger Davey: It is true that this will be the first commercial solar updraft tower, but we have carried out a lot of work in the lead up to this point. But all new technology needs incentives.

If the loan guarantee is still available and if we are able to provide information in time (we should be able to provide the information within a few months of ARUP starting its work), then we will certainly apply. We will also be eligible for the ITC and other incentives.

Enviromission spent the first 5-6 years of its life trying to develop the solar updraft tower in Australia. But you need political will and incentives, which are simply not available here. As a result of an “incentive desert” in Australia, we chose to move the technology to the most economically advantageous environment for building the towers, which at the time was the United States.

A key feature of solar updraft towers is that they are very site-specific; you need to take into account the geological landscape for the foundations, and the meteorological conditions. The site in Arizona is perfect, given that it is within close proximity to a major grid, it has flat land and a high insolation.

CSP Today: The technology requires a lot of land space and is highly visible. Can you provide some perspective on this and will this present a challenge/constraint for future projects? 

Roger Davey: It won’t present any constraints. The first plant is to be designed by ARUP. The different size of the plant is dependent on solar radiation and the size of the tower and collector is dependent on solar insolation and heat differentiation.

The first solar updraft tower will be roughly 750 metres tall and 130 metres in diameter. Its visibility is unlikely to cause concern, The Solar Tower will be an icon of engineering excellence (much like the Tour Eiffel/Eiffel Tower), and an icon of renewable energy, it will become a destination icon.

I don’t believe we will get much local opposition. The local constituents have already been very supportive. What we are proposing is a very big change of rural land use, and the project will employ 1,500 people during the construction period. 

We actually have two sites for two towers, so there will be even more job creation, going forward.

CSP Today: What are the key operational and maintenance challenges and or advantages of solar updraft towers and what is the expected average life-span of an updraft tower?

Roger Davey: The towers have a lifespan of around 80 years. The maintenance will really revolve around the collector, given that the tower won’t require  much maintenance. The turbines are pressure staged, with slow moving parts, so very low maintenance. They are more akin to jet engines and hydro turbines and nothing like traditional wind turbines.

The materials we use will be optimized for lightweight concrete construction and we have no doubts that the solar Tower can be built and that the technology is sound and robust; and in the words of Southern California Public Power Authority director Bill Carnahan ““SCPPA is excited to support a large scale solar technology that when successfully deployed could change the renewable energy landscape. The pricing and load profile of the Solar Tower coupled with its zero water power production cycle makes it a compelling alternative”.

To respond to this article, please write to the editor: Rikki Stancich