Chuck Kutscher

Development of a flow distribution and design model for transpired solar collectors

Unglazed, transpired solar collectors offer a low-cost, high-efficiency means for preheating outside air for ventilation and crop drying applications. Although large building wall applications for these collectors have generally performed well in the field, many have exhibited poor flow distribution which can prevent maximum efficiency from being achieved. The objective of this work was to develop a computer model which would run quickly on a personal computer and allow designers of transpired collectors to easily adjust geometric parameters to achieve reasonable flow uniformities and to determine efficiencies. This paper describes how this model was developed and includes results from model runs. In order to allow the model to run quickly on the PC, pipe network methods were used to develop a set of simultaneous equations in the unknown flow rates. Previous research results on heat exchange effectiveness, pressure drop, and wind heat loss were incorporated.

Water Use in Parabolic Trough Power Plants: Summary Results from WorleyParsons' Analyses

The National Renewable Energy Laboratory (NREL) contracted with WorleyParsons Group, Inc. to examine the effect of switching from evaporative cooling to alternative cooling systems on a nominal 100-MW parabolic trough concentrating solar power (CSP) plant. WorleyParsons analyzed 13 different cases spanning three different geographic locations (Daggett, California; Las Vegas, Nevada; and Alamosa, Colorado) to assess the performance, cost, and water use impacts of switching from wet to dry or hybrid cooling systems. NREL developed matching cases in its Solar Advisor Model (SAM) for each scenario to allow for hourly modeling and provide a comparison to the WorleyParsons results.Our findings indicate that switching from 100% wet to 100% dry cooling will result in levelized cost of electricity (LCOE) increases of approximately 3% to 8% for parabolic trough plants throughout most of the southwestern United States. In cooler, high-altitude areas like Colorados San Luis Valley, WorleyParsons estimated the increase at only 2.5%, while SAM predicted a 4.4% difference. In all cases, the transition to dry cooling will reduce water consumption by over 90%. Utility time-of-delivery (TOD) schedules had similar impacts for wet- and dry-cooled plants, suggesting that TOD schedules have a relatively minor effect on the dry-cooling penalty.

Life Cycle Assessment of Thermal Energy Storage: Two-Tank Indirect and Thermocline

In the United States, concentrating solar power (CSP) is one of the most promising renewable energy (RE) technologies for reduction of electric sector greenhouse gas (GHG) emissions and for rapid capacity expansion. It is also one of the most price-competitive RE technologies, thanks in large measure to decades of field experience and consistent improvements in design. One of the key design features that makes CSP more attractive than many other RE technologies, like solar photovoltaics and wind, is the potential for including relatively low-cost and efficient thermal energy storage (TES), which can smooth the daily fluctuation of electricity production and extend its duration into the evening peak hours or longer. Because operational environmental burdens are typically small for RE technologies, life cycle assessment (LCA) is recognized as the most appropriate analytical approach for determining their environmental impacts of these technologies, including CSP. An LCA accounts for impacts from all stages in the development, operation, and decommissioning of a CSP plant, including such upstream stages as the extraction of raw materials used in system components, manufacturing of those components, and construction of the plant. The National Renewable Energy Laboratory (NREL) is undertaking an LCA of modern CSP plants, starting with those of parabolic trough design. Our LCA follows the guidelines described in the international standard series ISO 14040-44 [1]. To support this effort, we are comparing the life-cycle environmental impacts of two TES designs: two-tank, indirect molten salt and indirect thermocline. To put the environmental burden of the TES system in perspective, one recent LCA that considered a two-tank, indirect molten salt TES system on a parabolic trough CSP plant found that the TES component can account for approximately 40% of the plant’s non-operational GHG emissions [2]. As emissions associated with plant construction, operation and decommissioning are generally small for RE technologies, this analysis focuses on estimating the emissions embodied in the production of the materials used in the TES system. A CSP plant that utilizes an indirect, molten salt, TES system transfers heat from the solar field’s heat transfer fluid (HTF) to the binary molten salts of the TES system via several heat exchangers. The “cold tank” receives the heat from the solar field HTF and conveys it to the “hot tank” via another series of heat exchangers. The hot tank stores the thermal energy for power generation later in the day. A thermocline TES system is a potentially attractive alternative because it replaces the hot and cold tanks with a thermal gradient within a single tank that significantly reduces the quantity of materials required for the same amount of thermal storage. An additional advantage is that the thermocline design can replace much of the expensive molten salt with a low-cost quartzite rock or sand filler material. This LCA is based on a detailed cost specification for a 50 MWe CSP plant with six hours of molten salt thermal storage, which utilizes an indirect, two-tank configuration [3]. This cost specification, and subsequent conversations with the author, revealed enough information to estimate weights of materials (reinforcing steel, concrete, etc.) used in all components of the specified two-tank TES system. To estimate embodied GHG emissions per kilogram of each material, two life cycle inventory (LCI) databases were consulted: EcoInvent v2.0 [4], which requires materials mass data as input, and the US Economic Input-Output LCA database [5], which requires cost data as input. IPCC default global warming potentials (GWPs) give the greenhouse potential of each gas relative to that of carbon dioxide [6]. Where certain materials specified in Kelly [3] were not available in the LCI databases, the closest available proxy for those materials was selected based on such factors as peak process temperature, and similar input materials and process technology. The thermocline system was modeled using the two-tank system design as the foundation, from which materials were subtracted or substituted based on the differences and similarities of design [7]. Table 1 summarizes the results of our evaluation. Embodied emissions of GHGs from the materials used in the 6-hour, 50 MWe two-tank system are estimated to be 17,100 MTCO2e . Analogous emissions for the thermocline system are less than half of those for the two-tank: 7890 MTCO2e . The reduction of salt inventory associated with a thermocline design thus reduces both storage cost and life cycle greenhouse gas emissions. While construction-, operation- and decommissioning-related emissions are not included in this assessment, we do not expect any differences between the two system designs to significantly affect the relative results reported here. Sensitivity analysis on choices of proxy materials for the nitrate salts and calcium silicate insulation also do not significantly affect the relative results.Copyright

Advances in Solar Buildings

In the autumn of 2002, 14 universities built solar houses on the National Mall in Washington, DC, in a student competition-the Solar Decathlon-demonstrating that homes can derive all the energy they need from the sun and celebrating advances in solar buildings. This paper describes recent progress in solar building technology that expands the designers palette and holds the potential to radically improve building energy performance. The discussion includes market conditions and solar resource data; design integration and modeling; window technology, daylighting, passive solar heating; solar water heating; solar ventilation air preheating; building-integrated photovoltaics; and solar cooling. The Solar Decathlon competition highlighted ways in which these strategies are integrated in successful solar buildings.

Heat Conduction of Inert Gas-Hydrogen Mixtures in Parabolic Trough Receivers

The annulus of a parabolic trough receiver is normally evacuated to prevent heat conduction between the internal absorber pipe and the external glass envelope. In the past, this vacuum has sometimes been compromised by hydrogen permeation from the heat transfer fluid through the absorber pipe. Heat conduction, and consequently receiver thermal loss, can be significantly increased by the presence of hydrogen in the annulus. Supplying receivers with inert gases in the annulus, or injecting receivers with inert gases after the vacuum has been compromised, could mitigate these heat losses. This study measures parabolic trough receiver heat conduction in the transition, temperature jump, and continuum regimes for argon-hydrogen and xenon-hydrogen mixtures at an absorber temperature of 350°C. Test results show that small heat loss increases over evacuated values are associated with the 95% inert gas/5% hydrogen mixtures and that from a performance perspective gas-filled HCEs would likely induce a 1–3% plant revenue decrease relative to evacuated receivers, but would protect against hydrogen-induced heat loss as long as there was sufficient quantity of inert gas in the annulus. Sherman’s interpolation formula predicted the inert gas and 95% inert gas/5% hydrogen mixture test results within experimental and model uncertainty, but did not accurately capture the larger hydrogen molar fraction test results. The source of this discrepancy will be further investigated.Copyright

Parabolic Trough Receiver Thermal Testing

NREL has fabricated a parabolic trough receiver thermal loss test stand to quantify parabolic receiver off-sun steadystate heat loss. At an operating temperature of 400°C, measurements on Solel UVAC2 and Schott PTR70 receivers suggest off-sun thermal losses of approximately 370 W/m receiver length. For comparison, a receiver from the field with hydrogen in its annulus loses approximately 1000 W/m receiver length. The UVAC2 heat loss results agree within measurement uncertainty to previously published data, while the PTR70 results are somewhat higher than previously published data. The sensitivity of several receiver performance parameters is considered and it is concluded that differences in indoor and outdoor testing cannot account for the difference in PTR70 thermal loss results.

Performance Evaluation and Outlook of Utility-Scale Linear Fresnel Technology

As one of the viable concentrating solar power (CSP) technologies, linear Fresnel collectors differ from parabolic troughs by virtue of their low-profile mirror arrays and fixed receiver assemblies. This technology is capable of achieving high concentration ratios and so is applicable to high-temperature solar power plant designs. In addition, its low wind profile and linear nature lead to low system and operation and maintenance (O&M) costs.In this report two linear Fresnel solar plant configurations, namely a direct steam generation (DSG) system and a direct high-temperature molten-salt plant, are examined via a levelized cost of electricity (LCOE) analysis. By treating LCOE as a function of the annual investment energy return (IER, or the ratio of annual net electricity to the total direct system cost) under various assumptions of O&M cost, a few plant scenarios employing high-temperature linear Fresnel technology are carefully configured to meet the aggressive LCOE goals of 8 cents/kWh and 6 cents/kWh. The latter is the Department of Energy (DOE) SunShot Initiative goal aimed at making CSP cost competitive in the current energy market. In particular, a linear Fresnel scenario with the potential to meet the SunShot goal is featured with a collector cost of

Chapter 13: Prospects for Renewable Energy

100/m2, an annual system energy efficiency of 18%, a storage system cost of

Current and Future Economics of Parabolic Trough Technology

15/kWh-th, and an O&M cost of

Design and Analysis of a Large Solar Industrial Heat Plant for Frito Lay in Modesto California

7.5/MWh. One of the most aggressive assumptions is an advanced power block with about 52% cycle efficiency and a turbine inlet temperature of 700°C.This work addresses unanswered questions regarding linear Fresnel cost and performance and identifies future research and development directions for linear Fresnel technology, including economic optimization of collectors and receivers, development of physical plant performance models, development of automated O&M mechanisms and sophisticated plant control software.Copyright