Electrical/Thermal Storage 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, transmission losses Avoided costs of transmission/distribution upgrades Power Quality – get.999+ availability with less pressure on grid + distribution, and reject/compensate dips, spikes, v. short outages, harmonics, reactive power

Electrical/Thermal Storage Opportunities (cont’d) Islanding potential enhanced: back-up energy and low-impedance inverter convenient for controlling frequency, clearing faults Ancilliary services to grid: stability enhancement, spinning reserve function

“Pure” Electrical Storage Technologies – Analysis of 10 kWh scale devices (May 2009 EECS290N Energy Storage Group Class Report) Cost effective electrical energy storage remains holy grail ! Flow and NaS technologies also of interest: MWh scale Costs still too high for general arbitrage appl

“Pure” Thermal Storage Pre-heating/cooling of working space Water/ice very dense storage media –Water has ~60 W-hr/kg for 50 degC swing –Ice-water heat of fusion ~ 0.1 kWhr/kg Established useful applications in pre-chilling for cooling and refrigeration Established applications in storing heat for space heating, hot water

Combined Opportunities: Main ideas: Water, mineral oil, and some salts (KNO3+NaNO3) are very low cost liquid media that can be directly interfaced with heat exchanger(s) 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

Combined opportunities (cont’d ) Ex. 2: Co-generation with thermal storage: Combustion-to meet electric demand (300 C ?) Thermal-Electric Conversion Thermal Reservoir(s) C Electrical output On Demand 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

Combined Opportunities (cont’d) Thermal Reservoir Waste heat stream C or higher Ex. 3: Waste heat recovery + thermal storage Heat Engine Converter Domestic Hot Water ? Electric generation on demand Huge opportunity in waste heat

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

Thermal System Diagram

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

Collector Cost Cost per tube [1] < $3 Input aperture per tube m 2 Solar power intensity G 1000 W/m 2 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, CITRIS Research Exchange, UC Berkeley, Spring 2007, also direct discussion with manufacturer [2] communications with manufacturer/installer

Cost Comparison Component$/W Collector0.95 Engine0.5 Installation -Hardware0.75 -Labor1.25 Total $3.45 Component$/W PV Module4.84 Inverter0.72 Installation -Hardware0.75 -Labor1.25 Total $7.56 “Rooftop” Solar ThermalPhotovoltaic Source: PV data from Solarbuzz

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.

Stirling Cycle Overview

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

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

Prototype 1: free-piston Gamma

DisplacerPower piston Temperatures: T h =175 o C, T k =25 o C Working fluid: 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 model254 W (thermal input) - 24 W (mechanical output)

Prototype Operation Power Breakdown (W) 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

Prototype 2 – Multi-Phase “Alpha”

Efficiency and Power Output Contour Plot 60Hz, 10bar Air