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
Displacer and power piston can independently be driven. 4 4

5. Prototype 2 – Multi-Phase “Alpha”

6. 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%

7. 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

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 8 8

9. 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

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
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. 

12. 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.

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 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) 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: 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 kWhe
In contrast a traditional system would cost $1.54- $3.64 per day with $0.30 per kWhe-hr electric cost
The corresponding savings per year would amount to ~$
The computed value includes use as a dispatchable source to opportunistically match peak prices.

19. 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

20. Comparison of Water Heating Options
“Consumer Guide to Home Energy Savings: Condensed Online Version” American Council for an Energy-Efficient Economy. August < >.

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

22. Thermal System Diagram

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

24. Stirling Cycle Overview4 1 2 3 24

25. 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.

26. Gamma-Type Free-Piston Stirling
Displacer
Power piston
Temperatures: Th=175 oC, Tk=25 oC
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 model 254 W (thermal input) - 24 W (mechanical output) 26

27. 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

28. 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

29. Collector Cost – no concentration
Cost per tube [1] < $3
Input aperture per tube m2
Solar power intensity G 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 years, 10 yrs warranty [2]
Easy installation – hr per module [2]
[1] Prof. Roland Winston, also direct discussion with manufacturer
[2] communications with manufacturer/installer 29 29

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