Beyond Zero talks to Milena Marinova of eSolar

Beyond Zero talks to Milena Marinova of eSolar, a company backed by Google, creating mid-sized fields based on concentrating solar thermal power tower technology.

Milena is Director of Strategy and Business Development for eSolar. Using small, modular, heliostats that reflect sun onto a tower-mounted receiver, eSolar makes cost competitive renewable energy.

eSolar began in 2007 and now has 1400 employees.

How does the eSolar system work?

The main difference between photovoltaics (PVs) and solar thermal is that in PVs, sunlight is converted to electricity immediately. Solar thermal creates heat or steam which is then converted to electricity.

How does a solar power tower work?

Typically a tower uses flat mirrors and the mirrors reflect and concentrate sunlight to a central tower. In that tower, steam powers a turbine to generate electricity. Esolar use multiple towers. (16 towers for a 46MW unit; 46MW is the electrical output).

It is a modular design. A tower with two subfields comprises a module. 16 of these modules fit into one quarter of a square mile of land (64 hectares). The modular design is enabled by the use of very small flat mirrors. These mirrors are less than four square metres in size. They sit on self-ballasted pedastals for support.

The mirrors track sunlight with high accuracy through an automated sun-tracking system. This enables the reflection of sunlight to the receivers on the towers; water is heated into steam; the steam from each tower is aggregated to a central power block; steam then powers a traditional turbine.

Ballasted pedastals sit on top of the ground and are low cost. It is able to be scaled to any size. It can be pre-fabricated. The mirrors are small and light. There is no heavy construction or foundation and no ground penetration so there is little environmental impact and the small compact design allows for the use of multiple smaller sites.

Efficiency?

Approximately one third (thermal output).

Cost competitiveness of smaller heliostats?

1. Smaller heliostats give economies of scale.
2. Automated installation and pre-fabrication means lower labour costs.
3. 46MW unit is a starting size and this means smaller land requirements and they can be closer to populations and are not environmentally damaging or interfering with agriculture.

How tall are the towers?

150 feet (50m), about the same height as wind towers.

Is there difficulty in cleaning the mirrors?

There is enough space between them so automated solutions are easy.

Water requirements?

This depends on the site and whether water or air cooled. Even a water-cooled solar plant uses little more water than a coal plant.

Energy cost for air cooling?

There’s a 10% reduction in efficiency but then water is saved.

Storage investigations?

Milena spoke of expertise in storage and stated that they were looking at what was economically viable and technologically reliable. She thought storage was not an immediate concern however, compared to utilities’ need to reduce peak load and to have solar supplement this. Currently the initial penetration of solar matches the requirements of the most expensive power requirements in the middle of the day. When solar becomes more dominant, storage options will be developed.