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Solar thermal and combined heat and power

Solar thermal and combined heat and power. Achintya Madduri, Mike He. Combined Opportunities:. Low-cost media – water, mineral oil, molten salts Heat engine ( eg . Stirling) provides high efficiency, eg . better than ~ 2/3 of reversible limit

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Solar thermal and combined heat and power

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  1. Solar thermal and combined heat and power Achintya Madduri, Mike He

  2. Combined Opportunities: • Low-cost media – water, mineral oil, molten salts • Heat engine (eg. Stirling) provides high efficiency, • eg. better than ~ 2/3 of reversible limit • Stirling converter enables excellent durability, cycle-ability (contrast with IC engine) Ex.1: Solar Thermal Electric System

  3. Stirling Engine • Can achieve large fraction (70%) of Carnot efficiency • Low cost possible for low temp design: • bulk metal and plastics • Simple components • Fuel (heat source !) Flexible • Reversible • Independent scalable engine and storage capacity • 25 kW systems (SES), MW scale designs proposed by Infinia

  4. Prototype 1: free-piston Gamma

  5. Prototype 2 – Multi-Phase “Alpha”

  6. Design Characterization

  7. CHP Design • Higher exit temperature (50 C) • Lower electrical efficiency • Higher system efficiency

  8. Collector and Engine Efficiency Collector with concentration G = 1000 W/m2 (PV standard) Schott ETC-16 collector Engine: 2/3 of Carnot eff. No Concentration

  9. Cost Comparison – no concentration Solar Thermal Photovoltaic With concentrator: expect substantial cost and area reduction due to efficiency increase Source: PV data from Solarbuzz

  10. Concentrator for Evacuated Tube Absorber • Conc. Ration C <= 1/sin(theta) • Can accept full sky radiation +/- 90 degrees on tubular absorber with aperture of Pi*D • Reduce # tubes by Pi • Insolation increased by ~Pi, results in substantially increased thermal efficiency and/or increased temperature

  11. Evacuated Tube Absorber

  12. Evacuated Tube Absorber

  13. Thermal Storage Example • Sealed, insulated water tank • Cycle through 50 C temperature swing • Thermal energy density of about 60 W-hr/kg, 60 W-hr/liter • Considering Carnot (~30%) and non-idealities in conversion (50-70% eff), remain with 10 W-hr/kg • Very high cycle capability • Cost is for container & insulator • Water to perhaps 200 C; mineral oil to 250-300 C

  14. Ex.: Co-generation with thermal storage Combustion-to meet electric demand (300 C ?) Electrical output On Demand Thermal-Electric Conversion Thermal Reservoir(s) 60 -100 C Thermal output on demand • One tank system: • cycle avg temp, or • thermocline • Two tank system Thermal-Electric conversion eff ~ >28% with high performance, longlife Stirling Converter

  15. Costs and Scale Potential of Distributed CHP • Thermal input to converter is perhaps 60-80% of combustion value without condensing heat exchanger, but perhaps >90% with condensing heat exchanger • Scale is substantial since 40,000 btu/hr thermal process in many homes translates to 13 kW thermal process, and to ~3 kWe generation at expected 25 % eff.: • 200M homes * 3kWe = 600 GWe

  16. Costs and Scale Potential of Distributed CHP Hot Water System • Cost Evaluation: • $14 per 1000 cubic-feet/1 million BTU/gigajoule • At 25% efficiency this translates to a pure electric cost of 20 cents per kW-hour • This electric generation comes with a bonus of 10,000 BTU of thermal energy per kWe-hr • Thermal Storage: • It take 35,000 BTU to heat a 60 gallon tank from 50° F to 120° F • For a reasonably sized, insulated water tank the loss due to conduction is 100 Btu/hr. Corresponds to a drop from 120° F to 115° F over 24 hours.

  17. Economic Analysis of CHP Hot Water System • For a Family of 4: 60-100 gallons/day of hot water. This requires 35,000-60,000 BTU of thermal energy which comes at a cost of 47,000-80,000 BTU/day ($0.66-$1.12 per day) with an electric production of 3.5-6 kWe • In contrast a traditional system would cost $1.54-$3.64 per day with $0.30 per kWe-hr electric cost • The corresponding savings per year would amount to ~$330-900 • The computed value includes use as a dispatchable source to opportunistically match peak prices.

  18. Electrical/Thermal Conversion and Storage Technology and Opportunities • Electricity Arbitrage – diurnal and faster time scales • LoCal market structure provides framework for valuation • Demand Charges avoided • Co-location with variable loads/sources relieves congestion • Avoided costs of transmission/distribution upgrades and losses in distribution/transmission • Power Quality – aids availability, reliability, reactive power • Islanding potential – controlling frequency, clearing faults • Ancilliary services – stability enhancement, spinning reserve

  19. Comparison of Water Heating Options “Consumer Guide to Home Energy Savings: Condensed Online Version” American Council for an Energy-Efficient Economy. August 2007. <http://www.aceee.org/Consumerguide/waterheating.htm >.

  20. Ex. 3: Waste heat recovery + thermal storage Waste heat stream 100-250 C or higher Thermal Reservoir Electric generation on demand Heat Engine Converter Domestic Hot Water ? • Huge opportunity in waste heat

  21. Thermal System Diagram

  22. Solar Dish: 2-axis track, focus directly on receiver (engine heat exchanger) Photo courtesy of Stirling Energy Systems.

  23. Stirling Cycle Overview 4 1 2 3

  24. Residential Example • 30 sqm collector => 3 kWe at 10% electrical system eff. • 15 kW thermal input. Reject 12 kW thermal power at peak. Much larger than normal residential hot water systems – would provide year round hot water, and perhaps space heating • Hot side thermal storage can use insulated (pressurized) hot water storage tank. Enables 24 hr electric generation on demand. • Another mode: heat engine is bilateral – can store energy when low cost electricity is available. Potential for very high cyclability.

  25. Gamma-Type Free-Piston Stirling Displacer Power piston • Temperatures: Th=175 oC, Tk=25 oC • Working fluid: Air @ ambient pressure • Frequency: 3 Hz • Pistons • Stroke: 15 cm • Diameter: 10 cm • Indicated power: • Schmidt analysis 75 W (thermal input) - 25 W (mechanical output) • Adiabatic model 254 W (thermal input) - 24 W (mechanical output)

  26. Prototype Operation

  27. Free-Piston “Gamma” Engine (Infinia) • Designed for > 600 C operation, deep space missions with radioisotope thermal source • Two moving parts – displacer and power piston, each supported by flexures, clearance seals • Fully sealed enclosure, He working fluid, > 17 year life • Sunpower (Ohio) has designs with non-contacting gas bearings

  28. Collector Cost – no concentration • Cost per tube [1] < $3 • Input aperture per tube 0.087 m2 • Solar power intensity G 1000 W/m2 • Solar-electric efficiency 10% • Tube cost $0.34/W • Manifold, insulation, bracket, etc. [2] $0.61/W • Total $0.95/W [1] Prof. Roland Winston, also direct discussion with manufacturer [2] communications with manufacturer/installer

  29. Related apps for eff. thermal conv • Heat Pump • Chiller • Refrigeration • Benign working fluids in Stirling cycle – air, helium, hydrogen

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