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Solar Thermal: Pros and Cons – Part 2: Concentrating Solar Power

RP Siegel | Monday May 21st, 2012 | 0 Comments

There is no perfect energy source. Each and every one has its own advantages and compromises. This series will explore the pros and cons of various energy sources.  Learn about other forms of energy generation here.

The term solar thermal has been used to describe two different types of systems. One is where solar panels are used to collect heat, which is used directly as domestic or process hot water, space heating, or in some cases, air conditioning. This most basic form of solar energy utilization, called solar heating and cooling (SHC), was covered in a previous post.

The other, very different type of solar thermal system involves concentrating solar collectors that focus the sunlight, amplifying its intensity, to achieve very high temperatures that can be converted to steam to drive an otherwise conventional thermal power plant or heat engine. This is generally called concentrating solar power (CSP), which is what we are going to discuss today.

According to the National Renewable Energy Laboratory (NREL), there are three basic types of CSP systems: linear concentrator, dish/engine, and power tower systems.

Linear concentrators are curved panels that reflect and focus the sun’s rays onto a tube that runs the length of the panel. The tube contains a fluid that heats up creating steam which is used to drive a turbine. The two main configurations are the parabolic trough type, where the tube runs along the focal line, and the linear fresnel type, which uses Fresnel lenses to collimate the reflected beam such that one receiver tube can be positioned over several mirrors. This type provides greater mobility in tracking the sun and is also less expensive.

Dish/engine systems

Dish engine systems are generally smaller in scale than linear concentrators, ranging from 3-25 kW. They consist of a circular parabolic disk mirror that track the position of the sun and focus its rays onto a power conversion unit which is located along an axis extending outward from the center of the disk at the appropriate focusing distance, which is based on the curvature of the disk. The power conversion unit consists of a thermal receiver and a heat engine. The thermal receivers absorb the heat reflected from the mirror and transfer it to the heat engine. The heat engine used most commonly is the Stirling engine. Stirling engines, unlike internal combustion engines, are heated externally, and do not require the internal explosions that characterize the engines under the hoods of most cars today. Because they can be heated by any of a number of energy sources, including concentrated solar, they are far cleaner and potentially far cheaper to operate than fossil fuel powered engines. Their biggest drawback is that they are slow to respond to changes, making them unsuitable for vehicular use (except perhaps in a hybrid system where they might be used to recharge a battery).

Finally, there are the perhaps the most impressive and most scalable systems, the power towers. Power towers, also known as point-focus central receiver systems, are similar to dish/engine systems, except there is one central engine surrounded by many dishes that all focus onto it. The heated receiver produces steam, which is then used to power a conventional turbine-generator. Two 10 megawatt projects were successfully demonstrated in the U.S., back in the 90s. Spain has also deployed several units up to 20MW (pictured), the most recent using molten salt for energy storage and transfer. In 2009, eSolar launched a 5MW power tower plant, utilizing 24,000 mirrors. The DOE is currently developing a 200-megawatt molten-salt solar receiver panel for power tower technology. Two very large scale plants are also scheduled for completion next year, in the U.S.: BrightSource energy’s 392MW Ivanpah facility on the California-Nevada border, and SolarReserve’s 110MW Crescent Dunes facility, northwest of Las Vegas.

More are on the way. Saudi Arabia recently announced plans to invest $109 billion in solar power, with an eye towards generating some 41 GW, up to a third of their projected demand for the year 2030. Details are not yet known, but it will almost surely involve power tower facilities.

Unlike solar PV and solar heating and cooling, which both take advantage of the fact that the sun shines everywhere and rely on an inherently distributed model, CSP, clearly adheres to the more traditional centralized utility model, where power is transmitted over long distances from a capital-intensive central utility, where it will be sold to ratepayers.

Concentrating solar power

Pros

  • Renewable. No fuels required.
  • Non-polluting. Carbon-free except for production and transportation.
  • Can serve as a drop-in replacement for conventional fuels to make steam
  • Operating costs are low
  • Can utilize thermal storage to better match supply with demand
  • High efficiency
  • Scalable to the 100MW+ level

Cons

  • Intermittent
  • Low energy density
  • Slightly more expensive than solar PV
  • Construction/installation costs can be high
  • Relatively new technology involved
  • Hard to compete against very cheap natural gas
  • They require a considerable amount of space
  • Some people find them unattractive
  • Manufacturing processes often create pollution
  • Heavily location dependent
  • Will involve significant transmission distances/losses

While expensive, and far from simple, CSP holds the promise of  clean energy source that can be produced at scales comparable to coal or nuclear, under the appropriate conditions. While still subject to intermittency, it has been suggested that improved thermal energy storage solutions may hold the key to the long term success of CSP.

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What about other energy sources?

[Image credit: Tom Raftery: Flickr Creative Commons]

RP Siegel, PE, is the President of Rain Mountain LLC. He is also the co-author of the eco-thriller Vapor Trails, the first in a series covering the human side of various sustainability issues including energy, food, and water in an exciting and entertaining format. Now available on Kindle.

Follow RP Siegel on Twitter.


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