A Solar Technology for Every Application

Acciona’s financing of Nevada Solar One, and a recent series of a financing, a prominent hire, and a big announcement from Concentrating Linear Fresnel Reflector (CLFR) developer Ausra has been keeping long-underappreciated Concentrating Solar Power (CSP) technology in the news recently.  I consider this great news, because the potential for cheap thermal storage of CSP and the gigantic size of the available resource means that CSP is likely to provide the backbone of reliability for any future decarbonized electric grid [Word Doc] where the clear skies which it requires to operate properly and sufficient transmission are available.

But CSP is only one of a broad range of Solar technologies, and here I will outline the framework which helps me understand and predict which ones are likely to be most successful.

To understand the future of any technology, you first need to understand its applications, which will lead to an understanding of the characteristics necessary to meet them.  Broadly, solar power is used to produce heat for climate control and process heat, and for electricity, both on the grid and off.

Daylighting

The oldest solar application is daylighting, the use of windows and other means allowing indirect sunlight to provide effective internal illumination inside buildings.  For individual homes, window and skylights are usually sufficient for the job, but there also exist architectural features such as light shelves and even active sun tracking systems which combine with fiber optics or mirrors [pdf]  to provide light to the interior of large buildings.  Such systems can provide significant energy and maintenance cost savings, as well as increase worker productivity.  They are particularly popular in schools because of studies which show enhanced student learning under natural light.

Thermal Applications

Solar thermal, when used for space heating is needed mostly in the winter in cold and temperate climates.  Because of the fact that it is only useful for part of the year, it needs to be simple and inexpensive to be practical.  Here, passive solar design and proper orientation of buildings is the hands down winner, because passive solar measures are inexpensive to free, with one of the most expensive steps being adding extra thermal mass, something which greatly enhances performance where daily temperature swings are large, and tends to remain fairly inexpensive given its low tech nature.   Passive solar design is almost certain to be a long term winner, although it is unlikely to be a big winner for investors because it does not require special products or materials.   Active solar thermal systems are typically too expensive to economically be used for only the part of the year when the heat is necessary, although when the heat from the system can be switched between multiple applications, such as domestic hot water or electricity generation, it can be economic for an active solar thermal system for at least part of a building’s space heating load.  

For process heat, which includes solar domestic hot water, as well as heat for industrial processes [pdf], the active solar thermal systems shine because year round usage can make these still relatively inexpensive systems easily economic.  These systems tend to be either flat plate collector systems, which circulate a working fluid under a black heat collector, or evacuated tube systems, which are somewhat more expensive, but can reach higher temperatures because the heat collector is a solid wire, which avoids problems with boiling the working fluid.  Solar parabolic trough systems are also sometimes used in large scale, high temperature industrial applications.

Electricity Generation

With electricity generation, both time and location become important.   Electric transmission is constrained by infrastructure, and and electric storage is often more expensive than the power being stored, leading to large price premiums for power delivered where and when it’s needed most.

The right place

For off-grid applications flat plate photovoltaic (PV) panels, which can be either thin-film or the more traditional crystalline silicon with a battery backup tend to be suitable despite the relatively high cost of power because of the scalability, relative simplicity, lack of moving parts, and low maintenance of the systems.  Concentrating photovoltaic (CPV) is seldome used in off grid homes to reduce up-front costs, because it tends not to work as well as flat plate collectors when there are clouds, and the need for a solar tracking system adds to maintenance costs which can be especially critical in the remote locations where off grid power is usually needed. Another form of practical off grid application is small scale power for lighting or equipment in areas where the grid is available but where the savings from avoided wiring make an investment in PV and a battery pack economical.  A common example of this are the now ubiquitous solar garden lights.

Photovoltaic technologies also have an advantage in distributed generation: placing the power source at the point of use.  The main advantage here is in their simplicity (which allows for low maintenance) and scalability, allowing the sizing of the power source to fit the need.  For instance, an electric utility might place west-facing PV on a transmission base station which is near capacity during times of peak load, thereby meeting a portion of that load and avoiding an expensive upgrade to the base station.

The right time

Since electricity typically requires expensive batteries for storage, technologies which can have inexpensive, built in storage have a cost advantage over ones that only produce power when the sun is s
hining.  Most solar electric technologies conveniently produce power on sunny summer afternoons, a time which normally corresponds to peak load in climates where air conditioning drives peak load.  This effect can often be enhanced by orienting the panels towards the west or southwest so that they are producing their greatest output in the afternoon.  This produces intermediate power, which is available when electric demand is high, but is also often available at non peak times, such as during the day in the winter.  Although such power is more valuable than other forms of intermittent power generation, which often have no correlation with the load profile, they also cannot be relied on to be available when needed, and are less valued by utilities which are responsible for providing power whenever customers want it. 

Dispatchable power is the most valuable form of generation (per kWh) on the electric grid, because the utility can use it only when demand is high and cannot be met with cheaper resources, while utilities also value base load power, which is almost always available and can be relied on at any time.  Since the sun is not always shining, these forms of power require some form of storage, and this means that they are best met with Concentrating Solar Power, which can be built with thermal storage, a much less expensive way to store power than batteries and other forms of electric storage (with the possible exception of Pumped Hydro, which is limited in its available capacity and location.)

Thin film vs. CPV

The incumbent photovoltaic technology, crystalline silicon is typically very expensive per watt, and there are two approaches currently being taken to cut costs: thin film and concentrating PV.  Thin film is another form of flat plate PV that requires much less and less specialized materials but typically has lower conversion efficiencies and durability than crystalline PV, which makes it inappropriate for applications that require a large amount of power generation in a small area, while concentrating photovoltaic (CPV) uses lenses or mirrors in to focus sunlight on small but very high efficiency cells to generate power at a lower cost.  CPV usually requires the ability to track the sun and few clouds, which means that it is unlikely to be as economic in distributed applications, although some companies are working to overcome these limitations.

Central Power Generation

For central power generation, the main factor in choosing between technologies is cost.  Here, the concentrating technologies (CSP and Concentrating PV) tend to have the advantage, and the ability to use transmission to bring the power to the point of use means that the generation can be placed in areas with a lot of sun and very few clouds where these technologies perform best.  The need for additional maintenance for solar trackers is less of an issue at a central solar plant, and this also give and advantage to the concentrating technologies.

Concentrating Parabolic Trough plants, Solar Tower, and Concentrating Linear Fresnel Reflector generators need large scale (in the hundreds of megawatts) to achieve their superior economics, while Dish Stirling and Concentrating photovoltaic (CPV) technologies achieve their economies of scale at less than a megawatt.  The superior scalability of Dish Stirling and CPV is largely negated by the cheap thermal storage (referenced earlier) available with the first three technologies which is not available with Dish Stirling or CPV.

Conclusions

Whenever a company announces a new technology with higher efficiency, lower cost, or better storage, it’s easy to get carried away and think that that one technology is destined to win out over all the others.  I hope you now appreciate that there are as many or more applications as there are technologies, and which technology has the upper hand will depend on the intended use.  When evaluating companies, it’s most important to consider the target market, and compare the technology to its true competitors.  This article and the following tables should provide a useful cheat-sheet when you do so.

If You Want to See it in Action

Next Saturday (October 6) is the National Solar Tour in the US.  Click here to find a tour near you and see many of these technologies in people’s homes.

Application Table

Electric Generation Technology Table

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