Matthew S. Orosz

Geometric Design of Scroll Expanders Optimized for Small Organic Rankine Cycles

The application of organic Rankine cycles (ORCs) for small scale power generation is inhibited by a lack of suitable expansion devices. Thermodynamic and mechanistic considerations suggest that scroll machines are advantageous in kilowatt-scale ORC equipment, however, a method of independently selecting a geometric design optimized for high-volume-ratio ORC scroll expanders is needed. The generalized 8-dimensional planar curve framework (Gravesen and Henriksen, 2001, “The Geometry of the Scroll Compressor,” Soc. Ind. Appl. Math., 43, pp. 113–126), previously developed for scroll compressors, is applied to the expansion scroll and its useful domain limits are defined. The set of workable scroll geometries is: (1) established using a generate-and-test algorithm with inclusion based on theoretical viability and engineering criteria, and (2) the corresponding parameter space is related to thermodynamically relevant metrics through an analytic ranking quantity fc (“compactness factor”) equal to the volume ratio divided by the normalized scroll diameter. This method for selecting optimal scroll geometry is described and demonstrated using a 3 kWe ORC specification as an example. Workable scroll geometry identification is achieved at a rate greater than 3 s−1 with standard desktop computing, whereas the originally undefined 8-D parameter space yields an arbitrarily low success rate for determining valid scroll mating pairs. For the test case, a maximum isentropic expansion efficiency of 85% is found by examining a subset of candidates selected the for compactness factor (volume expansion ratio per diameter), which is shown to correlate with the modeled isentropic efficiency (R2 = 0.88). The rapid computationally efficient generation and selection of complex validated scroll geometries ranked by physically meaningful properties is demonstrated. This procedure represents an essential preliminary qualification for intensive modeling and prototyping efforts necessary to generate new high performance scroll expander designs for kilowatt scale ORC systems.

Dynamic Modeling and Control System Definition for a Micro-CSP Plant Coupled with Thermal Storage Unit

Organic Rankine cycle (ORC) systems are gaining ground as a means of effectively providing sustainable energy. Coupling small-scale ORCs powered by scroll expander-generators with solar thermal collectors and storage can provide combined heat and power to underserved rural communities. Simulation of such systems is instrumental in optimizing their control strategy. However, most models developed so far operate at steady-state or focus either on ORC or on storage dynamics. In this work, a model for the dynamics of the solar ORC system is developed to evaluate the impact of variable heat sources and sinks, thermal storage, and the variable loads associated with distributed generation. This model is then used to assess control schemes that adjust operating conditions for daily environmental variation.Copyright

Technologies for heating, cooling and powering rural health facilities in sub-Saharan Africa

This paper examines technical and economic choices for rural electrification in Africa and presents the rationale for trigeneration (capability for electricity, heating, and cooling) in health and education applications. An archetypal load profile for a rural health clinic (25 kWh e · day−1 and 118–139 kWh t ) is described, and a regional analysis is performed for sub-Saharan Africa by aggregating NASA meteorological data (insolation, temperature, and heating and cooling degree days) using correlates to latitude. As a baseline for comparison, the technical, economic (using discounted cash flow) and environmental aspects of traditional electrification approaches, namely photovoltaic (PV) systems and diesel generators, are quantified, and options for meeting heating and cooling loads (e.g. gas-fired heaters, absorption chillers, or solar water heaters) are evaluated alongside an emerging micro-concentrating solar power (μ-CSP) technology featuring a solar thermal organic Rankine cycle (ORC). Photovoltaics hybridized with LPG/Propane and μ-CSP trigeneration are the lowest cost alternatives for satisfying important but often overlooked thermal requirements, with cost advantages for μ-CSP depending on latitudinal variation in insolation and thermal parameters. For a 15-year project lifetime, the net present cost for meeting clinic energy needs varied from 45 to 75 k USD, with specific levelized electricity costs of 0.26–0.31 USD kWh−1. In comparison, diesel generation of electricity is both costly (>1 USD kWh−1) and polluting (94 tons CO2 per site over 15 years), while LPG/Propane based heating and cooling emits 160–400 tons CO2 depending on ambient conditions. The comparative analysis of available technologies indicates that where the energy demand includes a mixture of electrical and thermal loads, as in typical health and education outposts, non-carbon emitting μ-CSP trigeneration approaches can be cost-effective.

Dynamic Simulation and Optimization of an Experimental Micro-CSP Power Plant

Small scale solar thermal systems are increasingly investigated in the context of decentralized energy supply, due to favorable costs of thermal energy storage (TES) in comparison with battery storage for otherwise economical PV generation. The present study provides the computational framework and results of a one year simulation of a low-cost pilot 3kWel micro-Concentrated Solar Power (micro-CSP) plant with TES. The modeling approach is based on a dynamic representation of the solar thermal loop and a steady state model of the Organic Rankine Cycle (ORC), and is validated to experimental data from a test site (Eckerd College, St. Petersburg, Florida). The simulation results predict an annual net electricity generation of 4.08 MWh/a. Based on the simulation, optimization studies focusing on the Organic Rankine Cycle (ORC) converter of the system are presented, including a control strategy allowing for a variable pinch point in the condenser that offers an annual improvement of 14.0% in comparison to a constant condensation pinch point. Absolute electricity output is increased to 4.65 MWh/a. Improvements are due to better matching to expander performance and lower condenser fan power because of higher pinch points. A method, incorporating this control strategy, is developed to economically optimize the ORC components. The process allows for optimization of the ORC subsystem in an arbitrary environment, e.g. as part of a micro-grid to minimize Levelized electricity costs (LEC). The air-cooled condenser is identified as the driving component for the ORC optimization as its influence on overall costs and performance is of major significance. Application of the optimization process to various locations in Africa illustrates economic benefits of the system in comparison to diesel generation.Copyright

Dynamic Simulation of Performance and Cost of Hybrid PV-CSP-LPG Generator Micro Grids With Applications to Remote Communities in Developing Countries

Energy infrastructure in rural areas of developing countries is currently deployed on an ad-hoc basis via grid extension, public and private sector solar home system (SHS) service using photovoltaic (PV) panels, and community distributed generation systems, also called mini or micro grids. Universal access to energy is increasingly pursued as a policy objective via e.g. the U.N. Millennium Develop Goals (MDG), Sustainable Energy for All (SE4All), and U.S. Power Africa initiatives. Rational allocation of energy infrastructure for 1.6b people currently lacking access requires a screening process to determine the economic break-even distance and consumer connection density favoring topologically diverse energy technology approaches. Previous efforts have developed approaches to determine grid-connection break-even distances, but work on micro-grid and SHS break-even distance and density is limited.The present work develops an open access modeling platform with the ability to simulate various configurations of PV, Concentrating Solar Power (CSP), and fueled generator backup systems with exhaust waste heat recovery. Battery and thermal storage options are examined, and typical meteorological year (TMY) data is combined with probabilistic and empirical load curve data to represent the appropriate physical dynamics. Power flow control strategy and infrastructure is optimized for a minimum tariff (USD/kWh) for cost recovery. Cost functions derived from manufacturers’ data enable performance and economic assessment for a case study micro grid in Lesotho.Copyright

MOMENT ANALYSIS OF A SCROLL EXPANDER USED IN AN ORGANIC RANKINE CYCLE

Recent studies on small-scale power generation with the organic Rankine cycle suggest superior performance of positive displacement type of expanders compared to turbines. Scroll expanders in particular achieve high isentropic efficiencies due to lower leakage and frictional losses. Performance of scroll machines may be enhanced by the use of non-circular involute curves in place of the circular involutes resulting non-uniform wall thickness. In this paper, a detailed moment analysis is performed for such an expander having volumetric expansion ratio of 5 using thermodynamic models proposed earlier by one of the present authors. The working fluid considered in the power cycle is R-245fa with scroll inlet temperature of 125 degrees C for a gross power output of similar to 3.5 kW. The model developed in this paper is verified with an air scroll compressor available in the literature and then applied to an expander Prediction of small variation of moment with scroll motion recommends use of scroll expander without a flywheel over other positive displacement type of expanders, e.g. reciprocating, where a flywheel is an essential component.

Chapter 16 - Solar thermal powered Organic Rankine Cycles

This chapter introduces the concept of solar energy as a source for driving Organic Rankine Cycle (ORC) power plants, and identifies relevant deployed examples. The nature of solar irradiance and its geographic dynamic is described, and various solar collector architectures and plant topologies are presented, including hybridization of solar with storage and alternative thermal resources, and the cogeneration possibilities of diverse applications including industrial and distributed generation scenarios. The intermittent nature of solar heat highlights the importance of dynamic simulation as a tool to understand and optimize solar ORC (SORC) systems and their control, and provides a basis for engineering and economic analysis of potential system designs.

Low-cost small scale parabolic trough collector design for manufacturing and deployment in Africa

Concentrating Solar Power is expanding its deployment on the African subcontinent, highlighting the importance of efforts to indigenize manufacturing of this technology to increase local content and therefore local economic benefits of these projects. In this study a design for manufacturing (DFM) exercise was conducted to create a locally produced parabolic trough collector (the G4 PTC). All parts were sourced or fabricated at a production facility in Lesotho, and several examples of the design were prototyped and tested with collaborators in the Government of Lesotho’s Appropriate Technology Services division and the National University of Lesotho. Optical and thermal performance was simulated and experimentally validated, and pedagogical pre-commercial versions of the PTC have been distributed to higher education partners in Lesotho and Europe. The cost to produce the PTC is 180 USD/m2 for a locally manufactured heat collection element (HCE) capable of sustaining 250C operation at ~65% efficiency. A version with an imported evacuated HCE can operate at 300°C with 70% efficiency. Economically relevant applications for this locally produced PTC include industrial process heat and distributed generation scenarios where cogeneration is required.

GROUND COOLING SYSTEM FOR IMPROVING THE EFFICIENCY OF LOW TEMPERATURE POWER GENERATION

This paper presents an analysis of an organic Rankine cycle (ORC) with dry cooling system aided by an earth-coupled passive cooling system. Several organic fluids were considered as working fluids in the ORC in the temperature range of 125-200 degrees C. An earth-air-heat-exchanger (EMU) is studied for a location in the United States (Las Vegas) and another in India (New Delhi), to pre cool the ambient air before entering an air-cooled condenser (ACC). It was observed that the efficiency of the system improved by 1-3% for the system located in Las Vegas and fluctuations associated with temperature variations of the ambient air were also reduced when the EAHE system was used. A ground-coupled heat pump (GCHP) is also studied for these locations where cooling water is pre cooled in an underground buried pipe before entering a condenser heat exchanger in a closed loop. The area of the buried pipe and the condenser size are calculated per kW of power generation for various working fluids.