Concentrators vs. Flat Plates

Concentrators are normally used on solar farms to make superheated steam that drive Sterling Engines or turbine generators used in standard power plants. Flat plate collectors are normally used for residential heating or solar hot water systems. Let’s explore the pros and cons of concentrator and flat plate solar collectors.

Concentrators focus parallel beams of light on a line or on a point. Tracking devices known as Heliostats are used to keep sunlight focused on a desired target.. A parabolic dish focuses light on a point, and a parabolic trough focuses light on a line. Let’s first look at some parallel dish systems to better understand how they work. As you can see from the above illustration Sterling Engines are used to generate power. They do this by responding to a difference in temperature. The sun must be tracked on a daily bases and seasonal adjustments are made every few days. These parabolic dish assemblies may work as an array or they may function individually, but they must track the sun to be of value.

It takes a large number of parabolic dish assemblies to make a practical solar power plant and the location a  power plant like this is also critical. 

Fortunately The South Western deserts in the United States provide the solar flux needed to generate electrical power from Sterling Engines. 

An alternative to individual dish assemblies involves the use of individual mirror assemblies focused on a tower that produces superheated steam.   The 10 MW Solar One and Solar Two heliostat demonstration projects in the Mojave Desert have now been decommissioned. This tower used molten salts to transfer heat to a boiler and produce super heated steam.  Brightsourceenergy has a tower that boils water at 500*C directly. It has approximately 2.6 Gig a watts of power under contracts with Southern California Edison and Pacific Gas & Electric Company. In addition, the company manages a 100,000 acre development site portfolio in California and the U.S. Southwest that has the potential to accommodate approximately 10 GW of installed capacity. 

OK so much for the parabolic dish that concentrates light in a small area to produce high temperatures. An alternative to the parabolic dish is the Parabolic Trough that concentrates heat on a pipe and transfers that heat to special oils that will not brake down at high temperatures. This oil is circulated through a kind of boiler where the heat is used to boil water and make steam. 

It takes a lot of troughs to generate enough Steam for a solar power plant. A tracking device is also required to keep sunlight focused on the pipe during the day. 

The equipment and labor needed to assemble a practical parabolic dish or parabolic trough solar power plant is beyond the budget of most homeowners, but if your heart is set on using concentrators to produce steam to produce electricity rather than photovoltaic panels a small set of parabolic troughs may be what you're looking for. This 5'x8' array in conjunction with a small steam engine is capable of producing up to2 K Watts of power on a sunny day. Some people also prefer using parabolic troughs to make hot water.

All of these things a possible but before investing a lot of time and money in a solar application you should consider the payback period as well as the life expectancy of the system.

My friend George has taken the time to compare the solar heat gathering ability of a parabolic trough compared to a flat plate collector. I am truly amazed with George's data but I am more amazed with George. He is a skilled craftsman and a first class engineer but there flat plates and there are flat plates and most flat plate collectors do not rely on a tracking system to gather heat so I would not automatically conclude that parabolic troughs gather more heat than flat plates. The spacing of the flow tubes on the absorber plate as well as the flow tube bonding method glazing material and other factors all have an effect on collector efficiency. There are some things we just need to discover on our own. 

Besides flat plate collectors and concentrators we also have evacuated tube systems. Evacuated tubes are in a strange land between flat plates and concentrators. They do not use a tracking device but most of them do make use of small stainless steel concentrators under each tube. Evacuated tubes are known for the high temperatures, but they do not always collect more heat. 

The thing that makes evacuated tubes special is their ability to hold on to the heat they collect. Flat plates collect more heat but much of the heat collected is lost through the glazing. So which is better? Generally speaking I would say if high temps are important go for the evacuated tube. If a high heat gain at temperatures below 130* F are all you need I recommend that you go for a flat plate collector. 

Remember it is more difficult to store heat at 180*F than it is to store heat at 130* F. The other thing to remember is that water above 110*F is not recommended for bathing. 

If you really want to save money on a solar hot water system I recommend building your own serpentine collector.

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