Thermionic Cascade Converter Concept for High-Efficiency Space Power

<div class="htmlview paragraph">The increasing need for higher electrical power levels for military and scientific space purposes has spurred development of energy conversion systems with higher efficiencies than the traditional solar array. Numerous nuclear options are currently under development. These fuels eliminate the need for large deployable solar arrays or concentrators, allow unrestricted pointing and maneuvering, and are inherently less susceptible to natural and threat environments; they are the only viable energy source for outer planetary exploration. Fission reactors are the most desirable nuclear source for electric power levels above 10 kW. Radioisotope sources have been used in conjunction with thermoelectrics up to 1 kWe for planetary exploration. However, for radioisotope systems in the 1 to 10 kWe range, a much higher conversion efficiency is required. For over 15 years, NASA and the Department of Energy (DOE) have been developing closed cycle turbo-alternator (dynamic) power conversion systems for space use. The Air Force and DOE are jointly developing such a system under the Dynamic Isotope Power System (DIPS) program.</div> <div class="htmlview paragraph">Two closed dynamic cycles are being evaluated for use as part of the DIPS system. They are the Closed Brayton Cycle (CBC) which uses an inert (single phase) gas as the working fluid. The other system is the Organic Rankine Cycle (ORC) which utilizes the phase change properties of the working fluid. Each has a significantly different effect on the design of a radiative heat rejection system, primarily because of the temperature variation of the CBC single phase heat rejection versus the nearly constant temperature two-phase (condensing) ORC heat rejection. The heat rejection system plays an important role, since the power conversion efficiency is strongly dependent on the heat rejection temperature. The lower the rejection temperature, the higher the efficiency. However, the radiator area requirement increases as the rejection temperature is reduced. On a weight optimization basis both systems are comparable, however, the ORC system requires more radiator area for the same cycle efficiency. With the design requirement that the radiator be mounted around the circumference of the vehicle, available area is limited. Therefore, the heat rejection system design may not be a clear-cut weight optimization but rather a matter of area reduction at minimum penalty to efficiency and weight.</div> <div class="htmlview paragraph">The paper presents the results of an optimization study performed on the heat rejection system for a space based ORC power system using an isotope heat source. The radiator sizing depends on the heat rejection temperature, radiator configuration, and radiator properties such as the fin effectiveness, emissivity, and absorptivity. The optimization analysis to evaluate the effect of each of these parameters on the system weight and area is presented.</div>

Microcell concentrating photovoltaics (µCPV) have the potential to improve performance and reduce the cost of solar power in space. Here, we introduce an ultracompact V-cone tailored edge ray (V-TERC) concentrator, rooted in nonimaging optics, which enables operation near the sine limit. Relative to previous space µCPV implementations, this concentrator design enables an approximate four-fold increase in concentration ratio for a given acceptance angle and specific power. We validate the design through ray tracing simulations and construction of a proof-of-concept system that consists of a 650 × 650 µm2 triple-junction microcell bonded to a 3.1 mm-thick prototype V-TERC optic. In outdoor testing on a sunny day, the system achieves a power conversion efficiency of 30% at a geometric gain of 137× with a specific power of 90W kg-1 and an acceptance angle of ±4.5°. This is a record combination for µCPV to date and represents an important step toward increasing efficiency and lowering the cost of solar power in space.

Energetic evaluation of a CO2 refrigeration plant working in supercritical and subcritical conditions

<div class="section abstract"><div class="htmlview paragraph">In today’s world, due to fast depletion of fossil fuel and the increasing CO2 emission, the need to switch to alternate energy sources are higher. Stringent norms on exhaust emissions in IC Engine vehicles implies, very complex after treatment systems. Already many OEMs have refined their development strategies towards phasing out of IC Engines and bringing in Fuel Cell vehicles, Battery Electric Vehicles and Hydrogen IC Engine vehicles. Focus is on Hydrogen for Long Haul vehicles. In this paper cooling system design is demonstrated for Fuel Cell, Battery and Power Electronics system in a Heavy Duty Fuel Cell Electric Truck. Radiator and Fans are selected based on the overall heat rejection and Coolant inlet temperature requirements of components. Cooling system circuit and pump is decided to meet the coolant flow rate targets. High temperature cooling system and Low temperature cooling system are explained in detail. Thermal simulation is done using simulation software KULI. Test results signifies good correlation.</div></div>

A review of optical errors and available applications of deflectometry technique in solar thermal power applications

Small satellites with increased capabilities in terms of power and propulsion are being demanded for future missions. This paper addresses an alternative bi-modal solution which consists of a solar thermal propulsion system coupled with a micro-Organic Rankine Cycle system, to co-generate thrust and electrical power. Current literature on bi-modal systems is limited to static power conversion systems such as thermionic conversion processes. Therefore, this paper expands the research of bi-modal systems to dynamic power conversion systems and latent heat storage systems. The paper documents the design process, key design parameters, and feasibility of this system for a Geostationary Transfer Orbit to Lunar Orbit insertion mission. The results of a single-objective optimisation show the system is most suitable on-board small satellites with a gross mass above 300 kg. The propellant accounts for 50% of the total system mass. The final design uses Silicon as the latent heat energy storage system due to its high specific energy of more than 250 Wh/kg. Additionally, the enthalpy method is used to describe the dynamic behaviour of the phase change material and results show the insulation thermal conductivity has the largest effect, up to 17%, on the receiver’s maximum achievable steady-state temperature.

Due to the growing world population and its increasing energy demand, the construction of new power plants is inevitable. At the same time, it is necessary to keep the rise of the global average temperature well below 2 °C compared to pre-industrial level in order to significantly reduce the impacts and risks of climate change. Therefore, an environmental friendly energy infrastructure is required to reduce greenhouse gas emissions. Solar thermal power plants are a promising technology, as they provide the possibility to buffer solar energy in a thermal storage. Thus, it can produce electricity according to the demand and is independent of meteorological fluctuations. A life cycle assessment is necessary to assess the environmental impact of a solar thermal power plant and its individual components. This work analyses the extent of the environmental effects of solar thermal tower power plants in the following selected impact categories according to the CML-method: Global Warming Potential (GWP 100 years), Abiotic Depletion potential (ADP elements), Eutrophication potential (EP) and Acidification Potential (AP). The most ecologically relevant components are identified and modelled with different specifications in order to compare the influence of different building types. Furthermore, the life cycle assessment model is applied to different geographical locations in order to enable site-specific considerations. The selected locations are Upington in South Africa, Evora in Portugal and Calama in Chile. The life cycle inventory is generated based on an extensive literature research and forms the foundation for the modelling of solar thermal tower power plants in the LCA software GaBi ts. As a result, the most ecologically relevant components are determined. A comparison of the different locations allows statements on the ecological potential of solar thermal tower power plants at a specific site. The heliostats and the molten salt (MS) as heat transfer fluid and storage medium dominate the impact on the GWP 100 years. The heliostats consist mainly of steel and glass and are needed in large numbers. The molten salt is used in a high amount and consists of a mixture from sodium nitrate and potassium nitrate which both have a high impact on GWP. Moreover, the transport and the construction on site have relevant contributions to GWP. The impact of the transport results from the large mass of all shipped components. The impact of the construction on site has copper cables as the main contributor. The end-of-life scenarios have a significant effect on the total impact. Depending on the recovery of the metals and especially the further use of the molten salt, the environmental impact is decisively influenced.

Optimal operation strategy of peak regulation combined thermal power units and concentrating solar power plant with energy conversion and spinning reserve

The double-effect vapour absorption refrigeration system working with HFC134a as refrigerant and dimethyl acetamide as absorbent is analysed. Cooling coefficients of performance in the range of 0·5–0·9 could be obtained depending on the operating conditions. However, sub-zero temperatures are difficult to obtain at typical heat source and heat rejection temperatures. Hence, cascaded systems are suggested for achieving low temperatures. © 1998 John Wiley & Sons, Ltd.

Organizing a closed Brayton cycle in such a way that the compression process is performed in the vicinity of the critical point where specific volumes are a fraction of those of an ideal gas yields performance indices particularly attractive, mainly at moderate top temperatures. Cycle thermodynamic analysis requires the development of adequate methods for the computation of thermodynamic properties above the vapour saturation curve about the critical point. Working fluids suitable for the proposed cycle can be found in the class of organics, in particular among the newly developed, zero ozone depletion potential, chlorine-free compounds. The numerous technical and environmental requirements which a fluid must meet for practical use combined with the peculiar thermodynamic restraints limit the number of suitable fluids. Mixing two substances of different critical temperatures yields an indefinite number of fluids with tailor-made thermodynamic properties. One such mixture 0.93 HFC23 + 0.07 HFC125 (molar fraction), having tcr = 30°C, at tmax = 400°C, pmax = 150 bar, gives an efficiency above 27 per cent with heat rejection temperatures between 89 and 33°C. With a different mixture composition with a 50°C critical temperature, at the same tmax and pmax, an efficiency of 25.1 per cent is attained in a combined heat and power generation cycle with heat available in the range 53-103°C. An experimental programme to test the thermal stability of organic fluids showed that top temperatures of 380-450°C are achievable with some commercially available fluoro-substituted hydrocarbons. In view of practical applications a conversion unit based on a reciprocating engine could handle without problems the pressures and temperatures involved. The use of turbomachinery would lead to power plant of large capacity for the usual rotor dimensions or to micro-turbines at high rotating speed in the low power range.

Up to 25 thermoacoustic cooler design parameters were optimized to provide high COP predictions in selected air conditioning applications. Capacities ranging between 300 and 1500 W were considered. Two different configurations were investigated: (1) a single stack with a Helmholtz resonator termination, and (2) two drivers and two stacks. The optimization scheme utilized the Simplex algorithm, and the analysis was performed by DELTAE coupled with an original heat exchanger modeling tool which incorporated conventional straight fin analysis and semiempirical flow correlations. Various requirements were imposed for the external cooling and heat rejection temperatures, ranging between 270 and 287 K on the cold side and between 310 and 320 K on the hot side. Finned tube heat exchangers with water as the secondary fluid were considered. The predicted values of COP were greater than 3.8 (0.36, relative to the Carnot value) for an application analogous to a small window air conditioner. The performance and characteristics of the two different configurations were compared, and the loss distribution in each case was calculated. Furthermore, the impact of various constraints was examined. The performance predictions are encouraging and suggest that thermoacoustic air conditioning may be a viable proposition.

In-orbit assembly mission for the Space Solar Power Station

A fresh look at space solar power: New architectures, concepts and technologies

Numerical investigation of the effects of the nano-enhanced phase change materials on the thermal and electrical performance of hybrid PV/thermal systems

This study involved an assessment of the potential economic benefit of small solar thermal electric power systems to small municipal and rural electric utilities. Five different solar thermal small power system configurations were considered in the study representing three different solar thermal technologies. The configurations included: (1) 1-MW, 2-MW, and 10-MW parabolic dish concentrators with a 15-kW heat engine mounted at the focal point of each dish. These systems utilized advanced battery energy storage. (2) A 10-MW system with variable slat concentrators and central steam Rankine energy conversion. This system utilized sensible thermal energy storage. (3) A 50-MW central receiver system consisting of a field of heliostats concentrating energy on a tower-mounted receiver and a central steam Rankine conversion system. This system also utilized sensible thermal storage. The approach used in determining the potential for solar thermal small power systems in the small utility market involved a comparison of the economics of power supply expansion plans for seven hypothetical small utilities through the year 2000 both with and without the solar thermal small power systems. Insolation typical of the Southwestern US was assumed. A comparison of the break-even capital costs with the range of plant costs estimated in this study yields the following conclusions: (1) The parabolic dish concentrator systems could be economically competitive with conventional generation if the lowest capital costs can be achieved. (2) The variable slat concentrator and central receiver systems would have to achieve lower costs than the lowest in the cost ranges generally assumed in the study to become economically competitive. (3) All of the solar thermal plant types are potentially more competitive in utilities which are heavily dependent upon oil.