This study describes preliminary optical analysis performed regarding a new collector called the Point Focus Fresnel Concentrator (PFFC). This collector combines the concepts of the linear Fresnel collector and central receiver systems to form a new concept of a focal point Fresnel concentrator with a dual-axis sun tracking system. It concentrates direct solar radiation using a number of flat mirrors positioned over a rotating frame. The frame tracks the sun in the azimuth direction, while each row of mirrors tracks the sun in the elevation direction, thereby allowing sunlight to be concentrated on the same point above the collector throughout the day. PFFC is considered suitable for a number of applications, such as power generation by concentrating photovoltaics (CPV) and Stirling engines, and process heat applications. In this study, the first attempt to characterize the optical performance of the collector is made. A prototype of the collector has already been built on the campus of King Saud University. It has a total footprint of 9 m 2 , and includes 900 mirrors, each of which is 7 cm × 7 cm. The receiver has a diameter of 10 cm. Optical performance is studied by ray tracing methods to obtain flux maps and intercept factors of the receiver. Results show that the average concentration ratio is in the order of 220 to 300 suns when mirrors with a 6-mrad optical error are used. For the same mirrors, the highest attainable average intercept factor (0.674) occurs in the winter due to the low particle loading in the atmosphere. When the optical error is reduced to 2 mrad, the average concentration ratio increases to 290 to 400 suns, and the average intercept factor increases to 0.892. In any case, if the current design of PFFC is to be used in conjunction with CPV, a secondary concentrator would be needed to achieve required concentration ratios in the order of 500 suns.

Energy storage is key to expanding the capacity factor for electric power from solar energy. To accommodate variable weather patterns and electric demand, storage may be needed not just for diurnal cycles, but for variations as long as seasonal. Five solar electric systems with energy storage were simulated and compared, including an ammonia thermochemical energy storage cycle, compressed air energy storage (CAES), pumped hydroelectric energy storage (PHES), vanadium flow battery, and thermal energy storage (TES). To isolate the influence of the storage system, all systems used the same parabolic concentrator and Stirling engine. For CAES, PHES and battery, the engine directly produced electricity, which was then converted and stored. For TES, heat transfer fluid was heated by the dish and stored, and later used to drive the engine to produce electricity. For ammonia, the dish heated an ammonia dissociation reactor to produce nitrogen and hydrogen, which was stored. Heat was recovered to drive the engine by reforming ammonia from the stored gases. Each system was simulated in TRNSYS with weather data for Louisville, KY and Phoenix, AZ with subsystem efficiencies and storage losses estimated from previous experimental results. All systems including the ammonia cycle involved time dependent storage losses. Losses from the receiver included convection and emitted radiation, both of which depend on receiver temperature. Overall (solar-storage-electric) efficiency of the ammonia cycle depended strongly on synthesis reactor temperature, ranging from less than 1% to ∼18% for both Louisville, KY and Phoenix, AZ, at 500 K to 800 K, respectively. In contrast, the effect of dissociation reactor temperature was less. Overall (solar-electric-storage-electric) efficiencies of the CAES, systems in the limit of zero storage time ranged from ∼10% to ∼18% for solar receiver temperature of 500 K to 800 K. The vanadium flow battery and PHES efficiencies ranged from ∼9% to ∼17% for the same conditions. TES initially provided 12 to 23% efficiency over the same range of temperature. When time-dependent storage losses were included, however, efficiencies for all systems declined rapidly except the ammonia cycle in both locations and PHES in Louisville. The ammonia system had the highest efficiency after one month of storage, an advantage that increased with time of storage. The simulations showed that TES was most efficient for diurnal-scale storage and the ammonia cycle for longer storage. Full capacity factor for solar electric production may be most efficiently accomplished with a combination of direct solar-electric production and systems with both diurnal and long-term storage, the proportions of which depending on weather conditions and electric demand profiles.

Thermal energy storage (TES) system integrated with concentrated solar power provides the benefits of extending power production, eliminating intermittency issues, and reducing system LOCE. Infinia Corporation is under the contract with DOE in developing TES systems. The goal for one of the DOE sponsored TES projects is to design and build a TES system and integrate it with a 3 KW e free-piston Stirling power generator. The Phase Change Material (PCM) employed for the designed TES system is a eutectic blend of NaF and NaCl which has a melt temperature of 680° C and energy storage capacity of 12 KWh. This PCM was selected due to its low cost and desired melting temperature. This melt temperature ensures the Stirling being operated at designed operating hot end temperature. The latent heat of this eutectic PCM offers 5 to 10 times the energy density of a typical molten salt. The technical challenges associated with low cost molten salt TES systems are the low thermal conductivity of the salt and large thermal expansion. To address these challenges, an array of sodium filled Heat Pipes (HP) is embedded in the PCM to enhance the heat transfer from solar receiver to PCM and from PCM to Stirling engine. The oversized dish provides sufficient thermal energy to operate a 3KWe Stirling engine at full power and to charge up the TES. The HP arrays are optimally distributed so that the solar energy is transferred directly from receiver to Stirling engine heat receiver. During the charge phase, the Stirling engine absorbs and converts the transferred solar energy to electricity and the excess thermal energy is re-directed and stored to PCM. The stored energy is transferred via distributed HP from PCM to Stirling engine heat receiver during discharge phase. The HP based PCM thermal energy storage system was designed, built, and performance tested in laboratory. The TES/engine assembly was tested in two different orientations representing the extremes of system operation when mounted on sun-tracking dish, horizontal and vertical. Horizontal represents the zero elevation at sun rise and the vertical represents the extreme of solar noon. The testing allows the examination of orientation effect on the heat pipe performance and the maximum charge and discharge rates. The total energy stored and extracted was also examined. The areas for further system refinements were identified and discussed.

Limiting solar power is the inability to cost effectively store energy. The most cost effective means to store solar energy is thermally in the ground, which can then be used for direct conversion to electricity. However, doing so is limited by a historically poor thermal efficiency of such engines. A novel Stirling engine is posed which more closely mimics a Carnot heat engine. It does this through the use of a new passive thermal ‘switch’ which permits heat flow into the expansion chamber of the Stirling engine only when the temperature of the chamber is above a desired value. Ideally heat would be added only at the end of the compression stroke and the beginning of the expansion stroke. Central to this thermal switch is the use of a vanadium dioxide (VO 2 ) low mass heat exchanger internal to the expansion chamber. This low mass heat exchanger allows the film material to track and react to the temperature changes within the expansion chamber, permitting it to transfer heat only when needed. An adiabatic model of this enhanced solar Stirling engine is developed. Results indicate that the thermal efficiency can be nearly doubled, delivering a second law efficiency of over 0.6. Further, a year round overall efficiency accounting for losses in the Stirling engine and solar thermal collectors of 7% appears to be feasible when this engine is integrated with ground solar storage, providing the necessary power to meet loads in a low energy residence. Such results demonstrate promise for future application of this technology.

The interaction of gases such as carbon dioxide (CO 2 ) and other so-called participating gases with thermal infra-red (TIR) radiation is one of the mechanisms behind global warming and climate change. The noticeable effect this apparently gives at atmospheric concentrations of around 400 ppmv can be made use of in technical systems where pure (and pressurized) CO 2 is confined in, for example, a double glass arrangement. Depending on pressure, temperature, gas composition and path length a certain “optical thickness” for TIR radiation is obtained that can be used to decrease or increase heat flows, or create temperature differences. The latter would allow for power production using, for example, a Stirling engine. Of great importance also is the temperature difference between ground-level surroundings and the sky. With significant amounts of electric power being used for air-conditioning, heating or cooling purposes a smart window set-up that makes use of the interference of TIR radiation with participating gases may result in significant reductions in energy use and costs. Typical applications can be found in residential and office building windows or a glass roof that covers large building structures like railway stations or parking areas. Besides glass, plastics may be used as window material. The purpose of this paper is to present the potential of energy recovery from TIR radiation using most importantly earth-to-space radiation of long wavelengths (> 4 μm), to be distinguished from incoming solar radiation at shorter wavelengths (< 4 μm). Most relevant here is the TIR absorption band for CO 2 around a wavelength of 15 μm. Note also that unlike solar irradiation the earth-to-space radiation is not limited to daytime and cloudless skies. Finally, some examples for technical system lay-out and performance are given. This gives an introduction by the paper by Fa¨lt and Zevenhoven submitted to this conference (Fa¨lt and Zevenhoven, 2010).

The research consisted in a conceptual and basic design of a prototype Stirling engine with the purpose of taking advantage of the solar radiation to produce electric energy. The work began with a bibliography review covering aspects as history, basic functioning, design configurations, applications and analysis methods, just to continue with the conceptual design, where the prototype specifications are determined. Finally, a basic dimensioning of the important components as heat exchangers (heater, cooler, and regenerator), piston, displacer and solar collector was elaborated. The principal conclusions were that the different analysis methods had dissimilitude between the results, in this sense, a construction of the prototype is necessary for the understanding of the complex phenomena occurring inside the engine.

Since many renewable energy technologies use low cost or free primary energy sources such as solar insolation or wind, the capital cost of conversion equipment can become the dominant factor in determining economic feasibility. A natural approach to lowering the capital cost per unit of electricity is to strive for high efficiency equipment, i.e., increase the amount of electricity produced. Another approach is to seek out low cost conversion technologies, i.e., lower the capital cost. Capital equipment costs must be significantly lower than currently available off-the-shelf technologies to make solar power generally attractive economically for small-scale electricity generation. One potential low capital cost energy conversion technology is the liquid piston Stirling engine. A necessary design component for liquid piston Stirling engines is estimation of the frictional losses in the oscillating liquid columns. While frictional losses for fully developed laminar and turbulent pipe flow are characterized quite completely, average frictional loss factors for the continually starting and stopping liquid flow in oscillating columns are less complete. Direct measurements of frictional loss using a log-decrement method are reported in the paper. Measurements were completed for a variety of piping and tubing sizes and configurations. It was found that liquid volume correlated damping coefficient data well. A comparison with an equivalent fully developed laminar flow damping coefficient is presented.

Sandia National Laboratories in conjunction with Stirling Energy Systems (SES) has increased the capabilities of the National Solar Thermal Test Facility’s Engine Test Facility to include a gas fired burner for advance testing on the Kochums 4-95 Stirling engine. SES is using this engine in its current solar energy production system and is performing a redesign for manufacturability and reliability. The gas burner will aid in this task allowing a safe, controlled test environment. A burner was developed for the SES engine and tested at Arizona University. The gas and air controls were scavenged from an old gas fired Stirling engine test performed in the 1990s. Goals for the burner are to have the ability to do attended tests, but then have unattended tests. Due to equipment limitations, the gas and air control skid did not have these capabilities and was modified with newer equipment such as a PLC controller. Software to run the burner efficiently was developed and the system is prepared for testing a Stirling engine.