Microgrid Module Compared to Solar Farms

Microgrid Module Compared to Solar Farms

I keep getting more questions on our Microgrid module. One question is how does a small Microgrid solar array with cement storage and ORC engine compare to Concentrated Solar Power (CSP) installations. See my previous blogs on what such a system is like and its economics.

The question comes from a simple fact: CSP solar farm electricity generation is 40% efficient. Solar energy heats steam to as high as 550C, then using the common Rankine Cycle steam turbine. “Carnot” thermodynamics (an ideal turbogenerator) shows that efficiency at these temperatures should be 40% or more.

Indeed, the U.S. Dept. of Energy says, “the current lowest-cost state-of-the-art commercial standard is estimated to be a central receiver configuration which utilizes a molten salt HTF [Heat Transfer Fluid], coupled with 10 hours of thermal storage, to deliver heat at ~550°C to a steam Rankine power cycle with a designed thermal-to-electric conversion efficiency of ~41%. As of 2013, this configuration was estimated to deliver an LCOE of approximately 13 ¢/kWhe without subsidies.”

Meanwhile an Organic Rankine Cycle (ORC) engine running at 300C gets only 20% efficiency: 20% of the sun’s heat is converted into electricity. How can a 20% efficient ORC engine compete with a steam turbine having twice the efficiency?

The answer is heat. We use the heat and they don’t. Either turbine – steam Rankine or Organic Rankine – produces heat as a result of the thermodynamic process of generating electricity. It’s called “low grade” heat because it has less value than the “high grade” heat the turbine uses. But low grade heat (heat with a temperature below boiling, 100C) is valuable for many processes. District heating (space heating of homes and businesses in cold climates), desalination (purifying salty or brackish water), absorption chilling (cool air for homes and buildings or make ice for transporting farm produce to market) and process heat (industrial heat for laundries, fabric processing and food processing) all use low grade heat.

The problem is that those efficient steam turbines can’t usually use the low grade heat they produce. With power in the 10 MW to 1000 MW range, these large installations typically need over 25 m2/kW of land area. The 100 MW Shams Solar Power plant (UAE) needed a site a mile on a side (2.5 km2) for its solar collectors. The 392 MW Ivanpah plant (USA) needed a site over 6 square miles (16 km2).

Most large CSP installations are in the desert where low diffuse radiation favors concentration with mirrors. These usually aren’t installations that are near a city. Large solar farms are sited far from population centers that could use the low grade heat they produce. Their low-grade heat is simply discarded. One method dumps the heat into rivers or the sea. Another uses cooling towers to dump low grade heat to the environment. If you’ve spent so much effort to capture solar energy, why throw most of it away?

By contrast, a 100 kW installation requires a half acre (0.2 ha) site for its solar collectors. Such a site can be near population centers that can use the low grade heat produced. For instance, a gated community in the U.S. can provide electricity to its homes while heating the same homes in the winter. An agricultural microgrid can run fans with its electricity while heating greenhouses at night. A rural microgrid in Egypt can pump water and power local villages day and night while desalinating salt water with its heat.

If the microgrid’s heat is used locally, its economics become more compelling. In my earlier economics blog, I showed that the heat energy revenue stream is about equal to its electrical energy revenue stream. Both energy forms deliver the same revenue stream. By using the heat, we have twice the savings with our microgrid compared to a desert solar farm. Doubling the savings makes up for the 2:1 efficiency difference between high temperature steam turbines and our lower temperature ORC turbine.

As I showed in the section on Microgrid Economics, the cost forecast of our system (Microgrid modules, concrete heat storage, ORC turbogenerator) is between $2.7/W (low cost labor) and $3.2/W (high cost labor) for only the electricity. By comparison, the Ivanpah solar farm cost $5.6/W for its electricity. The large solar farms can cost nearly twice what our Microgrid Module plant costs.

Capital cost is one comparison. Another is energy costs: the Levelized Cost of Energy or LCOE. By using the heat locally, we forecast an LCOE of electricity at $0.075/kW-hr. We get this low value by using the heat. We allocate half of the plant to electricity and half to heat – they both have the same revenue streams. As noted in my earlier blog, this is well below the $0.18/kW-hr LCOE of electricity from Concentrated Solar Power (see IRENA_RE_Power_Costs_2014_report at www.Irena.org), the category into which large solar farms fall.

Other advantages stem from a smaller installation. As already noted, small 100 kW plants are easier to site near population centers where their heat can be used. But these same plants can be installed by far smaller engineering firms. The 392 MW Ivanpah plant in Nevada USA was installed by Bechtel, one of the largest engineering firms in the US. Even a small engineering firm can install a 100 kW system. For smaller projects, costs are easier to finance, environmental impacts are less and approval times are shorter.

Our production methodology of sandwich fabrication means captive factory economics reign. Instead of buying solar collectors fabricated by a central factory, each solar factory makes its own solar collectors. Raw materials bought on the global market can save as much as half the costs of the system’s collectors compared to purchasing already-built collectors. Of course that’s why our LCOE is low: we take into account these savings.

Last is jobs. We bring jobs to a local community. Once a microgrid system is installed, those same factory people can produce FourFold modules that deliver heat and PV electricity at smaller scales. Solarize your community in the best way possible. Capture 70% of the sun’s energy instead of only 20%.