Concentrated Solar Power High Efficiency Engine

1. OVEVIEW

ARPA-E Contact: None.
Technical Subcategory: 1.2 Mechanical-Thermal.
Funding Request: $500,000 Funding Request.
Project Duration: 12 Month Project Duration.
Project Abstract: This is a high efficiency CSP (Concentrated Solar Power) engine prototype project for concentrated solar flux range of up to 16,000 watts. This uses a unique solar collector heat exchanger in an oven to minimized energy loss and a piston engine to allow a wide range of solar flux conversion as observed from dawn to dusk and for quick adaption to quick dynamic change in solar flux levels as by a passing cloud. The goal is to minimize heat and radiant energy loss with most of the collected energy passing through the piston engine for > 30% conversion efficiency and collecting the remaining energy as heat in a heat exchanger between exhaust and intake of a closed cycle engine. With a low heat loss of maybe 15%, the heat energy output is the balance of about 55% for 30% conversion efficiency, but has less heat output when the engine has higher thermal to mechanical conversion efficiency.

2. IMPACT

Energy is required for a productive economy. However most of the current energy used is from CO2 producing fuels stored in the ground, which continually adds pollution. Solar energy does not generate CO2 and collecting higher quantities of energy at higher overall efficiencies at valuable land and thus restricted physical spaces at convenient energy levels is highly desirable.

If this CSP engine is successful, it will provide an efficient engine for converting solar flux to both electricity and heat energy, which are important to residences, small businesses, and hotels which enhances economic and energy security of the USA 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 for 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, USA maintains a technical lead in developing and deploying advanced energy technologies.

3. STATE OF THE ART

Competing CSP methods of conversion of flux from personal sized solar collector (> 20 M2 area) to mechanical energy to drive an electric generator is using a steam turbine engine. The second solution is not to use concentrated solar energy and instead use low efficiency simpler solar cells.

The turbine engine method for CSP is undesirable because it may work well under the right conditions and with slowly changing solar flux conditions, but may work poorly under low flux conditions and rapidly changing flux conditions (as when a cloud passes in front of the solar flux source). A piston engine can operate well across broad power ranges as seen by the conventional piston engines. Turbine engine cars have been built (1950s, 1960s) but have been proven to not be able to compete with piston engine cars for efficiency and flexibility. Turbine engines only work well for narrow operation ranges. Solar cells with their 15% to 20% maximum energy conversion cannot compete with the potential 60% to 70% energy collection potential of a CSP.

4. INNOVATION

The new information for this project will show that a high quality, high efficiency CSP Solar Flux Heat Engine which will provide both mechanical power to run a power generator and also provide heat output for collecting the heat energy not converted to mechanical power.

CSP Motor Diagram

Figure 1: Solar Flux into Oven, Head > Heat Engine Design

This CSP engine works by mirrors concentrating solar photon flux focused into a small area which passes into the engine through a small window as shown in Figure 1: Solar Flux into Oven, Head & Heat Engine Design. The window uses 2 glass plates with a vacuum between to minimize heat loss from the oven through the window. A smaller window will reduce lost reflected light and blackbody photon flux out of the window, however currently targeting a 4” window. The oven has high quality insulation for minimal energy loss out of the oven insulation. Energy not lost from insulation loss and losses through the window, goes through the engine, which also has some small heat losses, but can be collected by the engine cooling system. Lost reflected flux and engine black body flux can be reduced by moving the window way from the head at the cost of larger oven insulation box surface area. Tradeoffs will need to be done.

Flux hitting the head made out of metal, typically has about 30% flux absorption, which the rest is reflected. Absorbed flux is converted to heat. This actually helps prevent the center head from absorbing energy in too small of an area and getting too hot and melting. The energy absorption in the center of head can be increased by adding some additional fins that will absorb additional energy from reflected flux. Energy not absorbed will bounce around until absorbed. The fin area toward the outside will cause a lot of localized reflections causing the majority of the remaining flux to be absorbed toward the outside of the heat exchanger, which heats the exchanger tubes below the head. The air in the oven will provide some energy conduction to more evenly distribute the heat energy. The heat conducts from the side of the heads down the copper heat exchanger tubes. Energy in the center of the head conducts down the center rod. The height and thickness of the heat exchanger tubes can be optimized for the particular energy flow range.

The piston engine is common 2-cycle mechanical design with all sealed bearing for crank and rod. The operation is well known, so it will not be discussed in detail, however the piston and cylinder maybe a combination of glass, ceramics and/or plastics to work with water as a piston lubricant. The piston compresses the cool air charge, amount controlled by the throttle value, into the hot heat exchanger head. The turbulence, large surface area, and high temperature quickly heats the air mass. As the piston starts moving downward, the hot air moves back into the cylinder and pushes on the piston as it moves down, converting the heat to mechanical energy.

A computer will control the amount of energy taken from the generator, the rate of air flowing through the engine, and quantity of steam injected. This has some control of the temperature of the head, temperature of the air on the expansion cycle, and can be used for some control of energy conversion efficiency, allowing efficiency to be dynamically optimized for the current incoming flux. This will allow maintaining the best engine efficiency during quick changes of input flux and for lower flux periods.

Primary thermal efficiency is determined by how much energy can be conducted into air-steam mix, by how hot the gases get, and energy removed during the expansion cycle. If less energy is being removed with less air, the head temperature will rise improving thermal transfer rates and mass temperatures, but there is a limitation to this since, melting of the head can occur. A component not shown is a low volume steam injection system used to inject energy into heat exchanger control volume. Steam heat exchanger tubes can wrap around the head, above the head, and at top of the interior of head to produce steam. A computer controlled injector injects small quantities of steam into the head when the piston is close to top dead center. Since small stream volumes are used, not much energy is used to pump the fluid to high pressures. The steam will add highly conducting molecules and also more molecules for conduction that will enhance transfer of the heat exchanger in addition to the energy added to the mixture from outside the direct thermal to air transfer of the air heat exchanger. Water molecules that do not transfer energy directly to air molecules will provide extra force to the piston during the expansion cycle. This increase of energy content of the mixture has potential to increase thermal engine efficiency at most of the operational power output levels. Unlike pure steam without continuous steam source, the mix will maintain compressible characteristics throughout the expansion cycle.

CSP systems using sterling engines have been created before. Using a low volume superheated stream generator with a long time conduction period, will allow energy outside of the air heat exchanger control volume, to be added into the mix for power generation, and will improve operation over a standard sterling air engine.

A CSP two cycle engine design with steam injection is a new technology innovation, so it advances the state of the art in thermal engine designs.

5. RISKS AND CHALLENGES

Technical risks include not having a CSP radiant flux source readily available which will mean testing with a non-solar flux energy source such as an electric heater or gas heater (such as a propane heater). This may not model the characteristics of concentrated solar flux as well as desired, and have reduced operational test accuracy. The aggressive heat exchanger may not be able to raise the air charge temperature fast and high enough in the short piston stroke time period to get high enough air temperatures for the expansion cycle. It is the temperature difference between hot and cold operation that determine the resulting thermal efficiency.

The new innovations include the solar flux to heat conversion heat exchanger design and using a 2 cycle engine designed with water (or similar) lubricated and cooled piston. A small glass window with a vacuum in the middle allows radiant flux into without allowing much conductive energy out. However, some reflected and black box radiation will be radiated out the window. The oven temperature will be hot which will help on engine efficiency but will be more difficult to keep from losing energy through the oven insulation. The reason for using water as a lubricant rather than oil as in traditional engine is prevent buildup of burnt oil on the heat exchanger, which lubricating is a complex issue of a 2 stroke engine design. The combination of air mixed with small quantities of superheated steam injected from outside the compression controlled surface may greatly change the characteristic of either air or steam operation and/or hold down the maximum mix temperature and/or hold too much energy when going out the exhaust port due to the air and water combined operation, but these are probably unlikely at all operation mix combinations, but is also the primary risk of this project. The first portion of the steam heat exchanger can also collect some heat from output of exhaust to improve system efficiency.

The techno-economic challenge is the cost of building a complete mirror, gimbal, thermal motor, generator system, and with high temperature water output, at a low enough cost that it makes economic sense. Over time, high volume cost of production of the thermal motor generator system with high temperature water output will become lower. The primary cost of the CSP system will probably be the mirror and gimbal system. Having a CSP system that is the right size for low-enough cost for the power output, but small enough for easy installation, and interesting enough for notability to generate interest in installation, will make this system relevant.

6. PROJECT PLAN

The objective is to design, build and test one or more prototypes based on this 2 Cycle Engine design including matched generator and single phase 240 tie inverter that power output can be computer controlled to set the load, to maximize efficiency of thermal engine for input energy. Characterize and document this engine type and determine overall thermal efficiencies.

The work will require mechanical engineering, electrical engineering, designing and building prototypes of engine, generator, and grid tie inverter. Some programming real-time engine and inverter programs. Some modeling of the engine thermodynamic operation and testing of broad range of operation energy ranges, improving models and documenting designs and results.

7. TEAM

The project will be managed and developed by Mike Polehn with over 30 years engineering experience, BS of Computer and Electrical Engineering, OSU (Oregon). Education includes mechanical engineering statics, dynamics, strengths, thermal dynamics, etc, electrical engineering, power, etc., and computer engineering. Will hire ME college students locally from WSU (Vancouver) part time and/or paid interns, to enhance their education experience.