Concentrating solar power (CSP) has potential to increase the amount of renewable energy on electric grids and reduce global carbon emissions, in particular because of its capability to incorporate inexpensive thermal energy storage. To realize this potential, development of latent heat storage with phase change materials (PCMs) is attractive because of its high energy density and low material cost. Research in this area has identified many promising materials, and has developed effective storage system designs that could be used to deploy PCMs. However, the field has lacked a rigorous methodology to select the optimal material for a particular CSP system. To address this need, here we present a holistic model that evaluates all important influences of PCM properties on the performance and cost of an entire CSP plant. We apply this model to over 100 of the most widely considered candidate materials, including both salts and metals. We quantify the relative value of each PCM property, and identify leading materials for application in CSP systems. This method and our results may aid future efforts to develop latent heat storage by providing a more informed view of how material selection impacts an entire CSP system.

In this work, we present the design and experimental results of a prototype latent heat thermal energy storage system. This prototype used 100 kg of aluminum-silicon as a phase change material with embedded heat pipes for effective heat transfer, a valved thermosyphon to control heat flow out of the thermal storage system, and a Stirling engine to convert heat to electricity. We tested this system for 11 simulated days of operation; each day included charging of the thermal storage tank, simultaneous electricity generation with heat input, and electricity generation from stored heat alone. On each simulated day, we set the engine to a different power level, allowing us to investigate the response of heat pipes and our valved thermosyphon to part-load conditions. The prototype demonstrated a maximum efficiency of 18.5% in converting stored heat to electricity, at a maximum power output of over 1 kW,. Extending these results to a commercial scale system with solar heat input, our modeling indicates that a discharge efficiency of over 30% and an annual efficiency of 18% could be achieved. This performance represents an advancement in established efficiency for any system that combines latent heat storage with electricity generation, and demonstrates that this system has potential for future commercial development.

In this paper, we present performance simulations and techno-economic analysis of a modular dispatchable solar power tower. Using a heliostat field and power block three orders of magnitude smaller than conventional solar power towers, our unique configuration locates thermal storage and a power block directly on a tower receiver. To make the system dispatchable, a valved thermosyphon controls heat flow from a latent heat thermal storage tank to a Stirling engine. The modular design results in minimal balance of system costs and enables high deployment rates with a rapid realization of economies of scale. In this new analysis, we combine performance simulations with techno-economic analysis to evaluate levelized cost of electricity, and find that the system has potential for cost-competitiveness with natural gas peaking plants and alternative dispatchable renewables.

Location selection is an important factor for the effectiveness and cost of a solar electricity generation system. Latitude and weather impact efficiency and availability, and local labor rates have a further influence on cost. To explore the impact of these effects, we performed techno-economic analysis of a small-scale solar power tower concept in six different locations in the United States, Australia, and India. The specific configuration we considered has a nominal power rating of 100 kWe and 5 hours of thermal energy storage. Further, we considered two deployment strategies: a single stand-alone 100 kWe tower, and a 20 MWe cluster of many towers. For each scenario, we used a probabilistic analysis method to evaluate levelized cost of electricity. We found that this configuration has potential for cost-competitive electricity generation and the ability to improve grid flexibility for integration of intermittent renewables. In addition, solar resource, latitude, and local labor rates all have an impact on the levelized cost of electricity of the system.

Latent heat thermal energy storage systems have the advantages of near isothermal heat release and high energy density compared to sensible heat, generally resulting in higher power block efficiencies. Until now, there has been no highly effective and reliable method to passively extract that stored latent energy. Most modern attempts rely on external power supplied to a pump to move viscous heat transfer fluids from the phase change material (PCM) to the power block. In this work, the problem of latent heat dispatchability has been addressed with a redesigned thermosyphon geometry that can act as a "thermal valve" capable of passively and efficiently controlling the release of heat from a thermal reservoir. A bench-scale prototype with a stainless steel casing and sodium working fluid was designed and tested to be reliable for more than fifty "on/off' cycles at an operating temperature of 600 degrees C. The measured thermal resistances in the "on" and "off' states were 0.0395 K/W and 11.0 K/W respectively. This device demonstrated efficient, fast, reliable, and passive heat extraction from a PCM and may have application to other fields and industries using thermal processing. (C) 2018 Elsevier Ltd. All rights reserved.

Modem electric grids that use intermittent renewables require energy storage to maintain reliability. Many potential solutions exist, but latent heat thermal energy storage shows particularly high potential for low cost grid scale energy storage. In this paper, we present the design and initial experimental results for a lab-scale prototype of a novel latent heat thermal storage system. This version of our prototype used 50 kg of aluminum-silicon alloy as a phase change material, and a novel valved thermosyphon concept to control heat flow from a thermal storage tank to thermoelectric generators for dispatchable electricity production. Our results validate the system: the thermal storage system was able to receive heat input, evenly distribute heat to and from the phase change material with small temperature gradients, and controllably dispatch heat to a heat engine for electricity generation on demand. With the basic principle of this technology demonstrated, our next step will be to evaluate and improve system efficiency.

In this work, we present the design, construction, and experimental results of a prototype latent heat thermal energy storage system. The prototype consists of a thermal storage tank with 100 kg of the aluminum-silicon eutectic as a phase change material, a valved thermosyphon that controls heat flow from the thermal storage tank to the power block, and thermoelectric generators for conversion of heat to electricity. We tested the prototype over four simulated days, where each day consisted of four phases of operation: charging, discharging, simultaneous charging and discharging, and storage. Our results show three major conclusions. First, the thermal energy storage system was able to receive and distribute heat with small temperature gradients less than 5 degrees C throughout the thermal storage tank. Second, the valved thermosyphon was able to effectively control heat transfer, demonstrating an on/off thermal conductance ratio of 430. Third, the interfaces between subsystems had small temperature drops: of the 560 degrees C temperature drop from the thermal storage tank to the heat rejection system, 525 degrees C occurred across the power block. This work overcomes the challenges of integrating previously-developed subsystems together, providing a proof-of-concept of this system.

We are developing a novel concentrating solar electricity-generating technology that is both modular and dispatchable. Solar ThermoElectricity via Advanced Latent heat Storage (STEALS) uses concentrated solar flux to generate high-temperature thermal energy, which directly converts to electricity via thermoelectric generators (TEGs), stored within a phase-change material (PCM) for electricity generation at a later time, or both allowing for simultaneous charging of the PCM and electricity generation. STEALS has inherent features that drive its cost-competitive scale to be much smaller than current commercial concentrating solar power (CSP) plants. Most obvious is modularity of the solidstate TEG, which favors smaller scales in the kilowatt range as compared to CSP steam turbines, which are minimally 50 MWe for commercial power plants. Here, we present techno-economic and market analyses that show STEALS can be a cost-effective electricity-generating technology with particular appeal to small-scale microgrid applications. We evaluated levelized cost of energy (LCOE) for STEALS and for a comparable photovoltaic (PV) system with battery storage. For STEALS, we estimated capital costs and the LCOE as functions of the type of PCM including the use of recycled aluminum alloys, and evaluated the cost tradeoffs between plasma spray coatings and solution-based boron coatings that are applied to the wetted surfaces of the PCM subsystem. We developed a probabilistic cost model that accounts for uncertainties in the cost and performance inputs to the LCOE estimation. Our probabilistic model estimated LCOE for a 100-kWe STEALS system that had 5 hours of thermal storage and 8-10 hours of total daily power generation. For these cases, the solar multiple for the heliostat field varied between 1.12 and 1.5. We identified microgrids as a likely market for the STEALS system. We characterized microgrid markets in terms of nominal power, dispatchability, geographic location, and customer type, and specified additional features for STEALS that are needed to meet the needs of this growing power market.

Conventional concentrated solar power (CSP) is a reliable alternative energy source that uses the sun's heat to drive a heat engine to produce electrical power. An advantage of CSP is its ability to store thermal energy for use during off-sun hours which is typically done by storing sensible heat in molten salts. Alternatively, thermal energy may be stored as latent heat in a phase-change material (PCM), which stores large quantities of thermal energy in an isothermal process. On-sun, the PCM melts, storing energy. Off-sun, the latent heat is extracted to produce dispatchable electrical power. This paper presents the design of a thermosyphon-based device with sodium working fluid that is able to extract heat from a source as demand requires. A prototype has been designed to transfer 37 kW of thermal energy from a 600 degrees C molten PCM tank to an array of 9% efficient thermoelectric generators (TEGs) to produce 3 kW of usable electrical energy for 5 h. This "thermal valve" design incorporates a funnel to collect condensate and a central shut-off valve to control condensate gravity return to the evaporator. Three circumferential tubes allow vapour transport up to the condenser. Pressure and a thermal resistance models were developed to predict the performance of the thermal valve. The pressure model predicts that the thermal valve will function as designed. The thermal resistance model predicts a 5500x difference in total thermal resistance between "on" and "off' states. The evaporator and condenser walls comprise 96% of the "on" thermal resistance, while the small parasitic heat transfer in the "off' state is primarily (77%) due to radiation losses. This simple and effective technology can have a strong impact on the feasibility, scalability, and dis-patchability of CSP latent storage. In addition, other industrial and commercial applications can benefit from this thermal valve concept. (C) 2017 Elsevier Ltd. All rights reserved.

We report on a new modular, dispatchable, and cost-effective solar electricity-generating technology. Solar ThermoElectricity via Advanced Latent heat Storage (STEALS) integrates several state-of-the-art technologies to provide electricity on demand. In the envisioned STEALS system, concentrated sunlight is converted to heat at a solar absorber. The heat is then delivered to either a thermoelectric (TE) module for direct electricity generation, or to charge a phase change material for thermal energy storage, enabling subsequent generation during off-sun hours, or both for simultaneous electricity production and energy storage. The key to making STEALS a dispatchable technology lies in the development of a "thermal valve," which controls when heat is allowed to flow through the TE module, thus controlling when electricity is generated. The current project addresses each of the three major subcomponents, (i) the TE module, (ii) the thermal energy storage system, and (iii) the thermal valve. The project also includes system-level and techno-economic modeling of the envisioned integrated system and will culminate in the demonstration of a laboratory-scale STEALS prototype capable of generating 3kW(e).

Anaerobic digesters convert organic waste (agricultural and food waste, animal or human manure, and other organic waste), into energy (in the form of biogas or electricity). An added benefit to bio-digestion is a leftover high-grade organic fertilizer. Models of the anaerobic digestion process do exist, but either rely on simple algebraic equations instead of biochemical reactions, or consider so many external parameters that they become overly complicated and require much input information and computation time. This work provides an intermediate kinetic model that predicts biogas output over time with few inputs. This kinetic model is justified by a small-scale laboratory experiment, and parameters are adjusted to match experimental results. This model can be used to optimize design parameters for an anaerobic digester, and provides information such as the relationship between digester sizing and feed rate. The process used here may be expanded to other feedstock materials and repeated for other similar applications, in an effort to expand anaerobic digestion systems as a clean energy source. Thesis Supervisor: Alex Slocum Title: Pappalardo Professor of Mechanical Engineering