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 Stirling converter enables excellent durability, cycle- ability (contrast with IC engine) Ex.1: Solar Thermal Electric System

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

Prototype 1: free-piston Gamma Displacer and power piston can independently be driven. 4 4

Prototype 2 – Multi-Phase “Alpha”

Design Characterization Design Specification Value Nominal Power Output 2.500 kW Hot Side Temperature 180 oC Cold Side Temperature 30 oC Design Parameters Pressure 30 bar Engine Frequency 20 Hz Regenerator Hydraulic Diameter 11um Heat Exchanger Hyd. Diameter (Air) 144um Regenerator Wetted Area 417.4 m2 Heat Exchanger Wetted Area (Air) 12.4 m2 Performance Non-Regenerator Flow Loss 108.6 W Regenerator Flow Loss 143.2 W Compression Loss 56.4 W Hot Side Heat Exch Temp Drop 2.81 oC Cold Side Heat Exch Temp Drop 3.30 oC Regenerator Effectiveness 0.997 Thermal-Mechanical Efficiency 21.7% Fraction of Carnot 65%

CHP Design Higher exit temperature (50 C) Lower electrical efficiency Higher system efficiency Design Characteristics Value Nominal Power Output 1.972 kW Thermal-Mechanical Efficiency 17.8% Fraction of Carnot 62% Hot Side Temperature 180 oC Cold Side Temperature 50 oC

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 8 8

Cost Comparison – no concentration Solar Thermal Photovoltaic Component $/W Collector 0.95 Engine 0.5 Installation -Hardware 0.75 -Labor 1.25 Total $3.45 Component $/W PV Module 4.84 Inverter 0.72 Installation -Hardware 0.75 -Labor 1.25 Total $7.56 With concentrator: expect substantial cost and area reduction due to efficiency increase Source: PV data from Solarbuzz

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

Evacuated Tube Absorber The operation of the solar collector is very simple. 1. Solar Absorption: Solar radiation is absorbed by the evacuated tubes and converted into heat. 2. Solar Heat Transfer: Heat pipes conduct the heat from within the solar tube up to the header. 3. Solar Energy Storage: Water is ciruclated through the header, via intermittent pump cycling. Each time the water circulates through the header the temperatures is raised by 5-10oC / 9-18oF. Throughout the day, the water in the storage tank is gradually heated. 

Evacuated Tube Absorber The heat pipes used in AP solar collectors have a boiling point of only 30oC (86oF). So when the heat pipe is heated above 30oC (86oF) the water vaporizes. Each heat pipe is tested for heat transfer performance up to 250oC (482oF) temperatures.

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

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

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

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.

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 kWhe In contrast a traditional system would cost $1.54- $3.64 per day with $0.30 per kWhe-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.

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

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 >.

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

Thermal System Diagram

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

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.

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 Power Breakdown (W) 26.9 10.5 0.5 15.9 1.4 5.2 9.3 Indicated power 26.9 Gas spring hysteresis 10.5 Expansion space enthalpy loss 0.5 Cycle output pV work 15.9 Bearing friction and eddy loss 1.4 Coil resistive loss 5.2 Power delivered to electric load 9.3 27 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

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 Solar-Thermal Collector Up to 250 oC without tracking [1] Low cost: glass tube, sheet metal, plumbing Simple fabrication (e.g., fluorescent light bulbs) ~$3 per tube, 1.5 m x 47 mm[1] No/minimal maintenance (round shape sheds water) Estimated lifespan of 25-30 years, 10 yrs warranty [2] Easy installation – 1.5-2 hr per module [2] [1] Prof. Roland Winston, also direct discussion with manufacturer [2] communications with manufacturer/installer 29 29

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