1428-1856: Concentrated Solar Power Generator

1. OVEVIEW

ARPA-E Contact: None.
Technical Subcategory: 2.5 Thermal-Radiant
Funding Request: $500,000 Funding Request.
Project Duration: 12 Month Project Duration.
Project Abstract: Collecting concentrated solar energy utilizes a broader wavelength spectrum of solar flux, has potential for higher efficiency, has multiple uses, and can generate both electricity and heat, and when designed for minimal energy loss results in much higher energy collection, as high as 70% efficient (30% electric and 40% heat), for a given solar area than the currently deployed solar cells which tend to have an efficiency of about 15%.

2. IMPACT

This addresses the problem of energy being a requirement of a functioning economy and the need to have more economical clean energy sources available, not detrimental to our world, for the required electrical and heating needs. This concentrated solar energy power generator will provide solar collection with characteristics that has superior power conversion, has multiple uses, can often be placed where solar panels cannot be easily used, and can have much lower installation costs than home solar cell panel systems.

If this concept is successful, it will provide a prototype of a self contained system for collecting usable solar power of both electricity and heat, which are important to residences, small businesses, and hotels which enhances economic and energy security of the US since it (i) Reduces imports of foreign energy from needing to import less heating oil. (ii) Reduces energy production greenhouse emissions since user does not need to burn heating oil or use the associated electricity from the power grid which major portions use greenhouse emitting generators. (iii) Improves the energy efficiency for home, business, and government installations that have both heat and electricity needs that exceed the heat and electricity output of the solar power unit. By developing and using this sophisticated highly efficient system, US maintains a technical lead in developing and deploying advanced energy technologies.

3. STATE OF THE ART

There are two methods of direct solar energy collection; solar cells and concentrated solar energy and a multitude of indirect methods through bio-fuels, (which will not be discussed further).

The low cost production solar cells tend to only convert a limited spectrum of light, rather than the full light spectrum, as well as the amount of energy generated which is often limited to a single electron per photon even for the higher energy photons which have a much higher energy. When compared to absorbed photons when converting to heat, energy is conserved and 100% of energy of each absorbed photon across the collection spectrum, is converted to heat.

Solar cells have low efficiency for a large area of collection resulting in large and bulky panels that have a large installation cost per usable watt and are very unsightly in land that is only used for solar power farms. Due to bulkiness and current methods used, the installation costs for a home owner's needs tend to be far too high per unit for wattage collected. Even if the solar panels cost $0, the current installation labor cost alone makes the power somewhat too expensive for effective economical deployment.

Solar cells only produce electricity when the actual needs of a home owner also includes heat for hot water and home heating. When hot water is needed and requiring to heat water from power obtained from solar cell power is a very inefficient solar heat collection method.

4. INNOVATION

We will learn that a self-contained concentrated solar power collection station, scaled for home needs with moderate efficiency, outside of basic mounting, electrical and hot water hook up can be economically built and deployed.

The solar power collection station works by using a large parabolic mirror mounted on a computer controlled motorized gimbal that automatically tracks the sun. The concentrated reflected light is reflected into a center parabolic mirror that further concentrates the collected flux into a small (<= 4") area at the base of the large mirror. Below the mirror is a moderate sized bay area for the easy install/removal and maintenance of equipment for collecting the solar flux. A course screen can be put over the large mirror to prevent birds and other animals from getting into the concentrated flux. Mirror washer subsystems keeps mirrors clean and tracking cameras look outward and also through a hole in the center mirror to monitor and give feedback to the tracking computer. The design can be streamlined in future work, towards being pleasant to look at, with a small 8" pipe mount point at the bottom, for placing on a pole 8'+ tall for safety.

The second part is a thermal electric power generator that has a window into a high temperature oven that converts the photons to heat on the head of the engine. The goal of the high temperature oven is to have a chamber to minimize loss of heat, reduced reflectance of collected phonons out of the oven, and reduced black body radiation out of the oven. There is tradeoffs between operating at a higher temperature, which gives better thermal efficiency (ƞthermal = (1 - TLow/THigh), T in Kelven, Carnot cycle) and heat loss from oven chamber insulation and black body radiation out of window. The heat provides power for running a piston engine which feeds a generator and inverter. Remaining exhaust heat can be collected and used for general heating. Example: SEGS solar collector high temperature 400C, using 30C low temp gives theoretical potential thermal efficiency of ƞthermal = (1 - TLow/THigh) = (1 - 303.15/673.15) = 54.97%.

Concentrated solar energy is not a new concept, however no progress in solar energy collection will be made, if no investment is made into progressing the performance and usability, which is the primary past failure of effective solar energy technology investment.

Two types of collected energy output is more valuable than just one type of energy. Large electric only power generation does not have effective use of the additional collected heat. However, a hotel has need of heating large quantities of water for showers and swimming pools, would have a valuable use of the additional heat for concentrated solar power units in its parking lot. Whatever heat is not converted to electricity is available as heat. With 30% electricity generation from the flux, the remaining heat out of the heat engine exhaust port, probably better than 40%, can be used for heating giving a overall 70% power collection efficiency.

This table shows the user yearly power output value of solar collection energy for competing solar cell panel installation and is compared with the proposed solar power station yearly value.

  PWatt PHeat ηGen ηInv

Peak

PKW

Norm

Flux

Daily

PKWh

Yearly

PKWh

ValueKWh

$0.10

Total

ValueKWh

4 KW of Panels 4000     0.80 3.20 6.14 19.65 7,172 $717 $717
12 M2 of Panels 2400     0.80 1.92 6.14 11.79 4,303 $430 $430
12 M2 CS (30%) 4000   0.95 0.95 3.61 6.89 24.87 9,079 $908  
12 M2 CS (40%)   4800     4.80 6.89 33.07 12,071 $1,207 $2,114

The first is a 4 KW power comparison followed by the same collector area comparison. The last two lines are the concentrator power station with one line for electric output and the second line is the heat output with equivalent resistive heat power. The standard solar inverter efficiency is low because of the low voltage resistive losses and need to use inductors with magnetic field storage and transfer, to pump the voltage up high. The power station is different, after the heat engine is a multiphase high voltage generator (~280V) to generate power to feed different type of inverter, that only needs to switch to a lower output voltage (240V), giving each component higher efficiency and better overall efficiency for given peak output. The Normalized Flux is from http://www.nrel.gov/gis/solar.html for Phoenix AZ area, with the solar cell value for tilted flat panel and the sun tracking concentrator directional flux of the sun, each is the average daily hours of 1.0 KW/M2 equivalent flux across the year. Finished with yearly power output & value.

It is easy to come up with cases where only the electricity is of value, however there are a large number of cases that both heat and electricity have value, as in hotels and homes, and it is in these cases that concentrated solar power has the most value. The table shows that a concentrator collector can have up to 3 times the real value of an equivalent 4 KW solar panel installation.

5. RISKS AND CHALLENGES

The major technical risk will be the high reliability double glass (like windshields) mirrors. The mirror will be a sectioned circular mirror with 4 identical mirrors on the outside, 4 identical mirrors on the inside area, all with about 1.5 meter perpendicular collection area (all bigger than 1.5 meters due to curved collection area), for total perpendicular 12 M2 concentration into the center concentrator about 1 meter in diameter, resulting in 3 unique mirror designs. Can a double glass sandwiched silver mirrors set be designed to get most of the flux into the 4" target area? Silver on top of glass can be done, but it will not have good durability. Can mirrors of sufficient quality for a 4" concentration be made at low enough costs for a low cost production product?

The innovation is the proof of concept to make an important technical capability and value obvious, so that deployment can occur. There is no unknown innovations since mirrors, thermal engines, generators, and inverters are well understood. The basic system will work, the difficulty will be in quality of engineering and components to reach the target system efficiency, which will help the US to keep a technological lead. With nominal 1000 watts per M2 below silver cutoff wavelength flux, to get 4000 watts, the overall efficiency of a 12 M2 collector would only need to be 30%. This includes loss in mirrors and glass windows, bird screen blockage area, heat loss in oven insulation, radiation loss from reflection out of oven, black body loss out of oven, thermal motor losses, electrical generator losses, and power output inverter losses. This requires quality and matched design across all the components of the full system.

The biggest techno-economic challenge is that most current solar dish systems are demonstration units, too small be useful for home use, or power grid generators that are too big, bulky, and costly, making each unit far too expensive for home, small business, and/or general use. The right size and cost can provide the economics and safety (small enough to put a bird screen over), for large volume deployment. Easily deployable systems can result in more constant business cash flows instead of the boom/bust/high risk conditions that destroyed Sterling Energy Systems in 2011. They had a very large system with a net 31.25% electric energy conversion which didn't have the additional post engine heat energy collection value, they went bankrupt. Large quantities of low cost systems can have better economics than large costly low volume systems.

A 4000 watt generator with a generous system price tag of $15,000 to $20,000 with much lower installation costs can easily beat the installed cost of a 4000 watt roof top solar panel installation. In full scale production, these could possibly have a much lower price tag. There is going to be more maintenance costs compared to flat panel solar cells, which also need occasional washing, but the added value of additional heat, more energy is collected per day, smaller ground area foot print, ease of use, potential for better efficiency, etc. outweigh the extra maintenance costs.

6. PROJECT PLAN

Design and build a concentrated solar collector with eventual target of 85% photon collection, 35% thermal engine efficiently (30% net), with a 15% thermal heat lost gives 40% addition heat collection for a total net of 70% solar energy collection performance. A 12 Meter2 concentrator with 1000 W/M2 flux, when conditions occur, would output about 4000 watts power and 4800 watts heat. These are not unreasonable high risk numbers, but it does require a well engineered system to achieve. However, this project will fall short of money and time to achieve the well engineered target, but a poorer prototype may have very good value towards moving forward.

Primary work focus will be on getting quality mirrors and basic gimbal working. A program to design the sandwiched glass double reflector mirrors, which will include refraction and multiple surface reflected images will be written. The objective is a 12 square meter collection area, perpendicular to axis, with a secondary center reflector to concentrate flux to 4" or less at the base of the primary mirror, with better than a 87% solar flux collection for photons with wavelenghts larger than the silver flux cutoff wavelength (~2 µm). Negotiate with glass companies to build prototype mirrors and determine production mirror costs. Due to the limited funds and short timeline, build low cost solar heat engine and power generator. Show that the oven temperature can easily support a 35% efficient or better thermal engine. The high quality engine, generator, and power inverter will be a stretch goal, so will probably be future work.

7. TEAM

The project will be managed and developed by Mike Polehn, BS of Computer and Electrical Engineering, OSU (Oregon). Education included mechanical engineering statics, dynamics, strengths, thermal dynamics , etc. I grew up on a very large orchard and learned many diverse things from farming. Spent considerable time working on and fixing different types equipment, modified engines with gear head friends, resulting in practical experience that would apply to creating components and building a prototype of this type of system.

We will hire ME collage students locally from WSU Vancouver, part time and/or paid interns to enhance their education experience.