They are among the most futuristic-looking power plants in existence. Row upon row of highly polished mirrors focus the sun's rays at a central point on a slender tower and generate electricity with no emissions and with no fossil fuel.

In practice, concentrating solar power technologies simply focus the sun's light energy and convert it into heat to drive what turns out to be a fairly conventional steam turbine generator or external combustion engine to generate electric power.

(Click to enlarge.) The heliostats provide the flash, but Ivanpah's workhorse may be its fairly conventional steam turbine technology, which is air cooled. Credit: BrightSource Energy(Click to enlarge.) The heliostats provide the flash, but Ivanpah's workhorse may be its fairly conventional steam turbine technology, which is air cooled. Credit: BrightSource EnergyThe novelty in part lies in the plant's use of thermal inertia that allow the technology to continue to generate electricity even after the sun sets. That feature can be extended--at a cost--by using molten salt or another energy storage medium to continue the steam conversion process for hours or even days.

This series of articles focuses on three primary technologies that are most commonly used to harness solar energy for electricity production: photovoltaics (PV), which directly convert light to electricity; concentrating solar power (CSP), which uses heat from the sun (thermal energy) to drive utility-scale turbines; and heating and cooling systems, which collect thermal energy to provide hot water and air conditioning.

This article will focus on CSP technology. Click here to read more about PV technology.

What Is CSP?

Unlike solar photovoltaic (PV) technologies, CSP plants use steam turbines, and can provide most needed ancillary services. Moreover, they can store thermal energy for later conversion to electricity. CSP plants can also be equipped with backup from fossil fuels to deliver additional heat to the system.

CPS plants typically consist of two parts: one that collects solar energy and converts it to heat (for example, the field of mirrors or "heliostats" that surround a central tower), and another that converts the heat energy to electricity (the conventional steam turbine generator or external combustion engine).

Schematic of a dish/engine system. Credit: DOE Energy Information Administration.Schematic of a dish/engine system. Credit: DOE Energy Information Administration.In the United States, CSP plants have been operating for more than 15 years. Almost all CSP projects require large areas for solar radiation collection when used to produce electricity at utility scale. As a result, most are sited in the desert Southwest and typically require long-haul transmission connections to reach load centers.

The 64 MW Nevada Solar One project, for example, requires 400 acres for its solar collecting array. A conventional natural gas-fired power plant of similar capacity might have a footprint not much larger than a store in a suburban strip mall.

CSP technology generally encompasses one of three approaches: trough systems, power tower systems, and dish/engine systems.

Trough Systems

These systems use large, U-shaped (parabolic) focusing mirrors that have oil-filled pipes running along their center, or focal point. The reflectors focus sunlight on the pipes. Doing so heats oil inside the pipes to as much as 750°F. The hot oil is then piped to an adjacent power plant where it boils water to make steam to run steam turbines and generators. In short, the solar array replaces whatever fossil fuel might have been used to produce steam.

A worker inspects the mirrors and pipe at ACCIONA's 64 MW Nevada One project near Las Vegas.A worker inspects the mirrors and pipe at ACCIONA's 64 MW Nevada One project near Las Vegas.One example of a utility-scale trough system is ACCIONA’s Nevada Solar One, which entered service in 2007. The $266 million plant has 64 megawatts of generating capacity and covers roughly 400 acres of desert some 35 miles outside of Las Vegas, Nev. The plant cost around $4,150 a kW to build and includes 760 parabolic concentrators with more than 182,000 mirrors.

These concentrate the sun’s rays onto more than 18,200 receiver tubes. The plant does not include any heat storage capability, but uses thermal intertia to generate electricity for around 30 minutes after sunset or after clouds move over the solar field.

Power Tower Systems

Power tower systems, also called central receivers, use many large, flat heliostats (mirrors) to track the sun and focus its rays onto a receiver. The receiver sits on top of a tall tower. The concentrated sunlight heats a fluid, such as molten salt, to as much as 1,050°F. This hot fluid can be used immediately to make steam for electricity generation or stored for later use.

Molten salt retains heat efficiently, so it can be stored for days before being converted into electricity. That means electricity can be produced during periods of peak need on cloudy days or even several hours after sunset.

Schematic of a power tower's basic components. Credit: DOE Energy Information AdministrationSchematic of a power tower's basic components. Credit: DOE Energy Information AdministrationOne of the world's largest power tower systems is the Ivanpah complex in California's Mojave Desert. The facility consists of three separate units: Ivanpah 1 has a total capacity of 126 MW, and Ivanpah 2 and 3 have a total capacity of 133 MW each. The $2.2 billion project was developed by BrightSource Energy and cost just under $6,000/kW to build. It was financed in part with $1.6 billion in Energy Department loan guarantees.

Dish Engine Systems

Dish/engine systems use mirrored dishes to focus and concentrate sunlight onto a receiver. The receiver is mounted at the focal point of the dish. To capture the maximum amount of solar energy, the dish assembly moves to track the sun across the sky.

The receiver is integrated into a high-efficiency "external" combustion engine known as a Stirling engine. A Stirling engine is a heat engine that operates by cyclic compression and expansion of air or other gas. The result is a net conversion of heat energy to mechanical work. Unlike an internal combustion engine (like the Otto cycle or the Diesel cycle), the heat energy source is external to the Stirling engine. In a typical solar application, the engine has thin tubes containing hydrogen or helium gas that run along the outside of the engine's piston cylinders and open into the cylinders.

Part of the 1.5 MW dish/engine array at the Tooele Army Depot in Utah. Credit: US ArmyPart of the 1.5 MW dish/engine array at the Tooele Army Depot in Utah. Credit: US ArmyAs concentrated sunlight falls onto the receiver, it heats the gas in the tubes. This causes hot gas to expand inside the cylinders. The expanding gas drives the pistons, which turn a crankshaft and drives an electric generator. The receiver, engine, and generator comprise a single, integrated assembly mounted at the focus of the mirrored dish.

The Tooele Army Depot Solar Project in Utah is a 1.5 MW dish engine system. It includes 429 dishes, each of which is 35 square meters in size and is capable of generating 3.5 kW of power. In service since 2013, the project is designed to meet roughly one-third of the army depot's annual energy demand.

CSP systems can eliminate the need to buy fossil fuels to generate electricity, and they produce no emissions. A chief drawback is that they typically require large swaths of land in generally isolated desert locations. They can have a negative impact on the local environment and may require transmission lines to reach load centers.

What's more, CSP facilities may not be fully dispatchable because solar energy is an intermittent resource. Energy storage is possible with power towers, but may be prohibitively expensive given the low cost of alternative technologies such as natural gas-fired combustion turbines.

Although the units are fairly expensive, they typically generate electricity at times of peak demand: hot, sunny days when air conditioning loads are highest.