Advanced Molten Glass for Heat Transfer and Thermal Energy Storage 0471-1528

Reply to Reviewer CommentsOverview: The reviewer comments had common themes in two major areas: 1) the feasibility of developing the glass material with desired properties, and 2) the viability of the viscosity pump concept. We have made significant progress since submitting the full proposal that reduces the risk from these two critical areas. See detailed replies below.

Progress in glass development: Our team has synthesized and screened new glass mixtures in our laboratory and has identified two novel mixtures that entirely eliminate the expensive molybdenum trioxide. The first contains sodium and phosphorous oxides and replaces the moly with three additional low-cost oxide materials. This mixture, called the “green glass” due to its deep emerald color, melts at 354 °C and was stable during a 2 hour furnace test at 1200 °C. Its heat capacity is 1.41 J/g-K, similar to molten salt. We believe that the green glass’s favorable thermophysical properties and lower cost lend credibility to our materials development approach.

We have identified a second glass candidate with very low viscosity, called the “black glass” due to its dark color. It consists of a completely different composition than the green glass. Its melting point is 494 °C, somewhat higher than the green glass. However at 500 °C the black glass appears very fluid, almost watery, based upon visual observations in an agitated crucible, with a viscosity lower than that of the green glass. The black glass was stable during a 14 hour furnace test at 1200 °C. The black glass will serve as a baseline from which to continue screening of high order mixtures to reduce its melting point while maintaining a low viscosity.

Our discovery of the low-cost green glass and the low-viscosity black glass increases our confidence of arriving at a single composition that achieves an optimal combination of low melting point, viscosity, and cost. See Figure 1 and Figure 2 for DSC and TGA test results of these materials. Receiving the award will allow us to continue to do finer screening in regions of interest as well as broaden the scope of screening.

Progress in viscosity pump development: We have conducted a proof of concept test of the viscosity pump with two proxy fluids in our laboratory. Based upon the results of this preliminary testing we have increased confidence that the viscosity pump is viable for fluids with viscosities varying from 1000 to 30,000 cP, a broad range likely to encompass the properties of the advanced molten glass. If we are able to reduce the viscosity below 1000 cP this would make pumping even easier by using a piston, gear, or other pump design similar to those used for pumping molten metals.

We constructed a simple prototype with a plastic rod immersed in the proxy fluid and spun by a hand drill. A test with 1000 cP silicone oil at room temperature showed good film thickness and adherence of the fluid to the rod. The fluid was skimmed off the rod by a stationary blade to generate flow. A test with 30,000 cP silicone oil showed similar results, with a somewhat thicker film produced on the rod. See Figure 3 for photos of the testing apparatus.

Assuming a high viscosity (30,000 cP) for the glass at 400 °C and using a 1” ID pipe 12 feet long results in a calculated pressure drop of 3.2 psi. This level of pressure is within the capabilities of the viscosity pump design according to the original patent document. The parasitic pumping load in our 5 kWt prototype is estimated to be less than 4%.

HALOTECHNICS, INC. Page 1 CONFIDENTIAL

Advanced Molten Glass for Heat Transfer and Thermal Energy Storage 0471-1528

Reply to other comments: Glass leakage in tank: We have had discussions with RHI Monofrax regarding the design of the hot tank with an internal lining of Monofrax bricks. They are confident that this design is feasible and have routinely used similar designs in many large scale glass production furnaces. Leakage through the joints of the Monofrax bricks will be reduced by the tight fitting tolerances possible with RHI’s production techniques. The tank will be constructed with close fitting bricks at ambient temperature. Upon heating, the bricks expand and lock into place, resulting in a close fitting design. Any leaks that do penetrate are likely to freeze before contacting the steel shell.

Receiver design: A receiver capable of operating at 1200 °C and compatible with the advanced molten glass is a critical part of the complete CSP system. The design of the receiver is outside the scope of the proposal, but we will identify the critical issues for receiver design as part of our proposed work with Pratt & Whitney Rocketdyne. PWR will select the alloys and joining techniques suitable for such an application. Issues that must be addressed include the thermal stresses in receiver tubes; the flow rate and pressure drops through the tube assembly; the oxidation stability of the tube alloy at 1200 °C; low cycle fatigue; and the reduction in radiative losses from the receiver at the proposed operating temperature. Selective coatings will likely be necessary to increase the absorptivity and reduce the emissivity of the receiver tubes. Achieving 1200 °C in a receiver is not a problem with existing heliostat technology. The challenge lies in developing the materials and designs capable of withstanding such a temperature for long enough to produce an economically viable receiver.

Freeze recovery: Modern central receiver CSP plants use a molten salt that freezes at approximately 240 °C. Our concept of a 1200 °C / 400 °C thermal storage system will leverage existing technology for freeze mitigation and recovery. The plants will be designed with a drain-down capability to reduce the risk of freezing overnight. All glass-wetted components will be centrally located near the tower and storage tanks. The tanks themselves will be designed with internal heaters capable of melting the glass inventory if necessary. Even without additional heating, the large volume of the storage tanks ensures that the temperature can be passively maintained above the freezing point of glass for many days. The prototype we will construct will be well-insulated and equipped with auxiliary heaters inside the tanks and at critical points in the glass piping to melt the glass in a controlled fashion in the event of a freeze-up.

Exergetic efficiency: Our proposed system design achieves a high round trip exergetic efficiency through several mechanisms. The tanks and pipes are well-insulated and lose very little heat. The discharge temperature from the hot tank is constant and is very close to the initial charge temperature. The high operating temperature of 1200 °C significantly increases the conversion efficiency of the power block receiving heat from the thermal storage system.

Engineering and chemistry team: The key personnel involved in the project have been identified in the full application, but we intend to hire 3-4 additional engineers with expertise in thermal system design and modeling who will perform much of the work on the project. Halotechnics is located near UC Berkeley, Stanford, and Silicon Valley, home to the nation’s premier talent pool for advanced engineering and materials science. Existing Halotechnics staff will be involved in the chemistry related work: Benjamin Elkin (experiment execution, chemist with degree from UC Berkeley); John

HALOTECHNICS, INC. Page 2 CONFIDENTIAL

Advanced Molten Glass for Heat Transfer and Thermal Energy Storage 0471-1528

Vaughn (analytical chemistry, inorganic chemist with over 10 years of experience in technology companies); Thomas Roark (lab manager, high throughput chemist); and Logan Esenther (supply chain analysis, industrial engineer from UC Berkeley).

Figure 1: (a) DSC test of green glass showing melting onset at 354 °C. (b) TGA of green glass showing stability to 1000 °C. Inset shows photo of green glass sample.

Figure 2: (a) DSC test of black glass showing melting onset at 494 °C. (b) TGA of black glass showing stability to 1000 °C. Inset shows photo of black glass sample.

30,000 cP1000 cP

Figure 3: Viscosity pump feasibility test at ambient temperature. Counterclockwise rotation at approximately 120 rpm. (a) 1000 cP fluid, (b) 30,000 cP fluid.

HALOTECHNICS, INC. Page 3 CONFIDENTIAL