Solar Thermal
Energy: Week 3
Michael Feula & Emily Shira
Fresnel Technologies
Line-focus geometry with one axis
Long, narrow, shallow-curvature (or flat) mirrors
Focus light onto linear receivers above mirrors
Receivers transfer energy to thermal fluid
One receiver shared between several mirrors
Focuses sun’s energy to ~30x normal intensity
Reduces costs compared to dishes or troughs
Aiming strategies used when suitable
Cheaper and more reflectors per area
Solar Thermal Power Plant Operation
Solar Thermal Power Plant Operation
Absorber heated to temperatures of 1000ºC or more
Open Volumetric Air Receiver
Ambient air heated to 650ºC-850ºC by reflected sunlight
Pressurized Air Receiver
Compressor pressurizes air to 15 bar
Air heated up to 1100ºC
Waste heat from gas turbine can go to heat boiler, drive a steam-cycle process
Reach efficiencies of over 50%
Energy Storage
•Highly impacted by fluids/molten salts
•Other storage mediums include:
• Water/ice tanks
• Heat pumps
• Heat storage in rocks, concrete, etc
• Heat storage in tanks
•Lots of room for innovation in this area
Fluids/Molten Salts
•Molten Salt- a salt that is solid at STP but becomes liquid with
heat
•Potassium Nitrate, Calcium Nitrate, Sodium Nitrate, Lithium
Nitrate, etc, used as an eutectic mixture
•Usually 60% Sodium Nitrate, 40% Potassium Nitrate
• Melting Point- 222° C
• Heat Capacity- 1.53 J/g.K
• Energy Density- 756MJ/m^3
•Compare to water: MP-100°C, HC- 4.184 J/g.K, ED- 250 MJ/m^3
•Another salt- 42% Calcium Nitrate, 43% Potassium Nitrate, 15%
Sodium Nitrate
Ice Tanks
•Makes use of large heat of fusion of water
•Heat of fusion=change in enthalpy resulting from heating a substance to
change its state from solid to liquid
•Ice produced during off peak hours used for cooling at later time
•Used in heat pump based heating systems
•Phase change energy provides thermal capacity near the bottom range
of temperature that water source heat pumps can operate in
•System to rides out the heaviest heating load conditions and extends the
time when source energy elements can contribute heat back into the
system.
Storage Application
•Solar energy is caught by a heat transfer fluid which heats to
almost 400°C, producing superheated steam
•This heat is used in part for power generation, and part is stored
in a molten salt storage tank
•The salt better retains heat than other storages
•At night, the molten salt is pumped back to the heat exchangers
where it releases its heat and returns heat to the now cold heat
transfer fluid
Resources
https://en.wikipedia.org/wiki/Concentrated_solar_power#Fresnel_reflectors
http://www.volker-quaschning.de/articles/fundamentals2/index_e.php
https://en.wikipedia.org/wiki/Thermal_energy_storage#Ice-based_technology