Numerical results pertaining to cyclic melting and freezing of an encapsulated phase change material (PCM), integrated into a solar heat receiver, have been reported The cyclic nature of the present melt/freeze problem is relevant to latent heat thermal energy storage (LHTES) systems used to power solar Brayton engines in microgravity environments Specifically, a physical and numerical model of the solar heat receiver component of NASA Lewis Research Center's Ground Test Demonstration (GTD) project was developed and results compared with available experimental data Multi-conjugate effects such as the convective fluid flow of a low-Prandtl-number fluid, coupled with thermal conduction in the phase change material, containment tube and working fluid conduit were accounted for in the model A single-band thermal radiation model was also included to quantify reradiative energy exchange inside the receiver and losses through the aperture The eutectic LiF-CaF2 was used as the phase change material (PCM) and a mixture of He/Xe was used as the working fluid coolant A modified version of the computer code HOTTube was used to generate results for comparisons with GTD data for both the subcooled and two-phase regimes While qualitative trends were in close agreement for the balanced orbit modes, excellent quantitative agreement was observed for steady-state modes

Modeling Cyclic Phase Change and Energy Storage in Solar Heat Receivers

This paper lays out an approach to develop Space Solar Power, with a small initial investment, a rising market, early revenue generation, and an evolutionary path to full Space-based power generation. The key idea is to use space assets initially for a global power exchange, establish the infrastructure, and then add space-based power as technology advances and revenue flows in. The system develops through 3 Phases, going from a Space Power Grid to an Augmented Grid to Full Space Solar Power. All conversion equipment is in low earth orbits, while ultralight collectors are in higher orbits. Large technical uncertainty persists in the efficiencies that can be obtained with beamed power. In the short term, efficiency may be only half that of the best terrestrial transmission, but the system can open markets that are now inaccessible. It also enables “green” power plants on earth. In the long term, as power from space dominates the system, the efficiency is better than today’s best terrestrial. Previous work on retail power beaming from space, and current ideas on small-scale terrestrial demonstrations, fit within the system. Within the context of a global enterprise, this route provides an economically viable path to full space solar power.

The Space Power Grid

Two sets of experimental data for cylindrical canisters with thermal energy storage applications were examined in this paper: 1) Ground Experiments and 2) Space Experiments. A 2-D computational model was developed for unsteady heat transfer (conduction and radiation) with phase-change. The radiation heat transfer employed a finite volume method. The following was found in this study: 1) Ground Experiments, the convection heat transfer is equally important to that of the radiation heat transfer; Radiation heat transfer in the liquid is found to be more significant than that in the void; Including the radiation heat transfer in the liquid resulted in lower temperatures (about 15 K) and increased the melting time (about 10 min.); Generally, most of the heat flow takes place in the radial direction. 2) Space Experiments, Radiation heat transfer in the void is found to be more significant than that in the liquid (exactly the opposite to the Ground Experiments); Accordingly, the location and size of the void affects the performance considerably; Including the radiation heat transfer in the void resulted in lower temperatures (about 40 K).

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Experimental and Computational Investigations of Phase Change Thermal Energy Storage Canisters

Visual and photographic observations of unidirectional solidification of liquids that shrink by about 10%% is reported. The liquid is contained in a constant-volume (sealed) cylindrical cell. To estimate the effect of gravity, cooling is provided from above or below. For degassed liquids, when solidified from below, it is observed that the shrinkage is accommodated at the top (like a cavity). However, when solidified from above, the shrinkage is accommodated by formation of voids and worms filled with vapor. The effect of dissolved gases is also investigated. It is observed that dissolved gases tend to distribute the voids for both solidification from below and above. Hence, dissolved gases may be used to control the location of shrinkage voids. Some discussions and predictions are made regarding the location of voids formed under zero gravity.

Void Formation in Unidirectional Solidification

This paper considers the conditions, opportunities and constraints under which beamed power might be attractive for space-based enterprises. First, an evolutionary, revenue-generating architecture for beaming electric power through space to terrestrial and space-based customers is presented, to understand the likely environment where full-scale Space Solar Power will become a reality. 12The Space Power Grid approach trades the large risks inherent in the launch cost reductions required for GEO-based microwave SSP concepts, against the nearer-term prospects of advances in millimeter wave and infrared laser conversion, beaming and reception. Space is envisioned as a venue for real-time power exchange between terrestrial power plants. With a constellation of 100 millimeter wave relay spacecraft in 2000km sun synchronous orbits trading with 200 ground stations, the system is shown to break even inside 17 years with fairly realistic parameters, facilitating a revenue-generating scale-up to full SSP. Conditions needed to break even by 50 years from project inception at a level of 3.4 Terawatts are presented. The logic of beaming to space-based customers is explored, concluding that this becomes viable mostly where high power levels are needed, implying either power-intensive commercial enterprises in orbit, or beamed propulsion.

Implications of inter-satellite power beaming using a Space Power Grid

A thermal analysis was performed of a thermal energy storage canister of a type suggested for use in a solar receiver for an orbiting Brayton cycle power system. Energy storage for the eclipse portion of the cycle is provided by the latent heat of a eutectic mixture of LiF and CaF2 contained in the canister. The chief motivation for the study is the prediction of vapor void effects on temperature profiles and the identification of possible differences between ground test data and projected behavior in microgravity. The first phase of this study is based on a two-dimensional, cylindrical coordinates model using an interim procedure for describing void behavor in 1-g and microgravity. The thermal analysis includes the effects of solidification front behavior, conduction in liquid/solid salt and canister materials, void growth and shrinkage, radiant heat transfer across the void, and convection in the melt due to Marangoni-induced flow and, in 1-g, flow due to density gradients. A number of significant differences between 1-g and o-g behavior were found. This resulted from differences in void location relative to the maximum heat flux and a significantly smaller effective conductance in 0-g due to the absence of gravity-induced convection.

R. P. Wichner

Papers

Thermal analysis of heat storage canisters for a solar dynamic, space power system