Solar Energy Part 3: Thermo-solar San Jose State University FX Rongère February 2008

Thermo-solar Absorption of the solar radiation Heating of a fluid to transfer energy to a process Differences come from the type of solar collectors and from the type of working fluid

Thermodynamics The state of a system of pure material at the equilibrium is determined by two variables: Energy: E, e (lower case per unit of mass) Entropy: S, s Energy balance: conservation – Closed system Entropy balance: no conservation – Closed system First law Second law

Definition of the Entropy Ludwig Boltzmann’s grave in Vienna

Thermodynamics “Thermodynamics is a funny subject. The first time you go through it, you don't understand it at all. The second time you go through it, you think you understand it, except for one or two small points. The third time you go through it, you know you don't understand it, but by that time you are so used to it, it doesn't bother you any more.” “The second law of thermodynamics holds, I think, the supreme position among the laws of Nature. If someone points out to you that your pet theory of the universe is in disagreement with Maxwell’s equations - then so much the worse for Maxwell’s equations. If it is found to be contradicted by observation, well, these experimentalists do bungle things sometimes. But if your theory is found to be against the second law of thermodynamics I can give you no hope; there is nothing for it but to collapse in deepest humiliation.

Thermodynamics (2) Open system (mass transfer) Energy balance Entropy balance

Thermodynamics (3) Material characteristics provide relations with Pressure and Temperature Ideal gas: Incompressible fluid:

Heat exchange Energy balance:

Efficiency oCoC oFoF Efficiency (%)

Absorber Up to 30 o C (80 o F) – Swimming pools

Flat-plate Collector Up to 80 o C (150 o F) – Home heating, Water heating Sometimes vacuum is made in the collector to prevent convection  Glass decreases radiation and convection  Insulation is usually of polyurethane foam or mineral wool, sometimes mineral fiber insulating materials like glass wool, rock wool, glass fiber or fiberglass are used.

All technologies use selective surface coating to: Maximize solar radiation absorption Minimize collector emission Selective surface coating Black chrome, black nickel, and aluminum oxide with nickel (galvanization) Titanium-nitride-oxide layer, (steam in vacuum process) Wave length λ, μm Emissivity, Absorptivity, ε λ, a λ

Evacuated Tube Collectors Over 80 o C (150 o F) up to 160 o C (300 o F) – Industrial porcess

How a heat pipe works? Evaporation process capture more energy by mass unit than one-phase flow. Example, water and steam: Lv=2,260 kJ/kg, Cp=4.18 kJ/kg/K required flow is about 50 times lower than for water heating Size is smaller, loss are reduced The flow is maintained in natural convection if the collector is angled over the horizontal

Pool heating Sizing the collector and the pump ASHRAE ch. 4 ASHRAE ch. 48

Step1: Heat transfer CIRCULATION ADDITION

Step2: Heat transfer modeling Radiation heat transfer a Σ =.8 to.9 ε Σ =.8 to.9 Source: R Siegel, J R. Howel Thermal Radiation Heat Transfer Hemisphere Publishing 1981

Step2: Heat transfer modeling Convective heat transfer Source: ASHRAE Application Handbook p 49-2

Step2: Heat transfer modeling

Evaporation Source:ASHRAE Application Handbook p 4-6

Dry and Moist Air Dew Point Kg water/kg air Saturation pressure of air: Pa=Humidity Ratio*Atm Pressure

Step2: Heat transfer modeling Conduction in the ground k cond : Conductivity of the soil (1 to 3 W/m/K – 0.6 to 1.8 Btu/hr/ft/ o F)

Step3: Balance Equation m circ = Masse of water divided by 8 hours C = Thermal capacity of the water (4,180 J/kg/K, 1 Btu/lb/ o F)

Step4: Sizing the collector Max Temperature elevation: 8 o F Solar radiation: Ex: facing south with 30 o of tilt

Step4: Sizing the collector Efficiency:.8 Flow rate: Required energy: Required area: Resource: Units: SI

Step 5: Simulation If no shade and good wall insulation, solar radiation absorbed by the pool easily compensates the heat loss by evaporation and convection: Absorbed solar radiation: 280 kWh/day

Without Solar Collector Temperature gain in a day is about 4.5 o C for 13 o C, 3 o C for 18 o C and 2 o C for 24 o C.

With Solar Collector Temperature gain in a day is about 10 o C for 13 o C, 8.5 o C for 18 o C and 7 o C for 24 o C. Addition: 300 kWh/day Savings compared to gas heater: 10 Therm/day $ 12/day Cost: $ 100/m2 $ 4,500 Use: 60 days/year Pay back: 6 years

With Solar Collector South-East orientation provides faster heating in the morning but less overall energy. Addition: 290 kWh/day

Step 6: Installation Typical installation of a solar collector for pool heating:

Step 7: Sizing the pump and pipes Some orders of magnitude hp