Week 8 - Solar Energy

Overview

  • Solar fundamentals

  • Solar PV

    • Basic function

    • Types of PV

    • Installed capacity in Australia and Globally

  • Solar Thermal

    • Types and global installation

Solar radiation

  • Sun is a big fusion reactor

    • Fusion is when you have incredibly high temp and pressure strong enough to force hydrogen and tritium atoms together to create helium and releasing energy/ The energy is released is in accordance with E = mc2.

    • Fission reaction occurs when uranium or plutonium radioactively decays to other elements, releasing huge amounts of energy according to this equation but it is a much simpler reaction to sustain. So current nuclear reactions are fission reactions.

  • The core of the sun is incredibly hot, the surface of the sun is significantly cooler like 6000 Kelvin.

  • The amount of energy emitted by the sun can be calculated using the Stefan-Boltzmann law

    • j= σT4

    • where the energy is the Boltzmann constant times temperature to the power of four

    • The amount of energy released by a body and all bodies releases energy in this way, not just hot bodies and is proportional to the temperature to the power of four so is very sensitive to changes in temperature.

Solar fundamentals

  • Solar radiation originates from the sun and arrives at the Earth at 1360 Wm-2 (top of the atmosphere)

    • There’s a nice piece of algebra to show this (see Moodle)

    • Once the radiation propagates through space from the sun when it arrives at the top of the Earth’s atmosphere before it passes through the atmosphere, we get about 1360W/m2. That’s a significant amount of energy.

  • Averaged over the Earth’s surface (taking into account clouds etc) we get an average of 270 Wm-2 (about 180 Wm-2) in Melbourne

  • Solar radiation is described as shortwave, around 500nm

Natural Greenhouse Effect

  • Wavelength of radiation function of the temperature (Wein’s law):

    • λ = b/T

  • Solar radiation is centred around 500nm

  • Terrestrial radiation at 10-15μm

  • The atmosphere is largely transparent to solar radiation, but absorbs terrestrial longwave radiation

Describing the position of the Sun in the sky

  • To calculate the clear sky incoming radiation we need:

    • latitude and longitude

    • zenith (deviation from vertical)

    • azimuth angle

    • depth of the atmosphere

    • angles of the solar collector

  • zenith angle

    • if the sun was directly overhead the zenith angle would be zero

    • If the sun is on the horizon or just rising or setting then the zenith angle is 90 degrees

  • azimuth angle

    • deviation from the south

    • it can also be north depending on which hemisphere you are and what you define at it

    • normally in the southern hemisphere we use north as being zero for the azimuth angle

    • when looking at the west where the sun sets, you’ve got an azimuth of -90 degrees.

    • when the sun is rising in the east you have an azimuth of positive 90 degrees


  • The lower the zenith angle, the less atmosphere the radiation has to pass through before it gets to the surface

  • Not only do you get more watts per square metre because the radiation is perpendicular to the surface but you get more radiation because of less atmosphere

  • When the sun is very low in the sky on the horizon, you get less watts per square metre because the radiation is coming in at quite an angle compared to the surface. Also radiation is lost through interactions with the much deeper amount of atmosphere the radiation has to pass through.

  • We also need to know a bit about the angles of our solar collector to know how much radiation its going to collect.

Diffuse and direct radiation

  • Solar radiation arrives at the Earth’s surface in two forms - direct normal irradiance (DNI) and diffuse horizontal irradiance (DHI)

  • Total is referred to as Global Horizontal Irradiance (GHI) = DHI + DNI*cos(Z) (where Z is the zenith angle)

  • PV can utilise both direct and diffuse

  • Concentrating Solar Thermal only uses the direct



  • Direct normal irradiance, which is the radiance that is coming directly from the sun. The direct beam that we would look at if we look directly at the sun and we also have the diffuse horizontal irradiance.

    • The diffused radiance is what we get when the sun that the solar radiation is interacts with the atmosphere, atmospheric particles refract the atmosphere and affect the radiation meaning it comes in from lots of different directions.

  • If we add the two together we get the global horizontal irradiance

  • Photovoltaics can utilise both direct and diffuse radiation

  • But the concentrating solar thermal can only be used for the direct part because its focusing the sun’s radiation onto a focal point

  • PV panels will work perfectly well on a cloudy day when you’ve got a lot more diffuse radiation and a lot less direct beam but there is less radiation available, but they still will take advantage of that diffuse radiation.

    • They may operate at 50% of their capacity on a sunny clear day.

  • Most people will orientate their panels because they want to maximise how much power they produce

  • Moving towards, systems with your electricity bill, where you tend to pay more for electricity during the peak period (between 4pm and 8pm)

    • In this case you might want to orient your panels towards the west, towards the setting sun

  • Things like fixed, variable and feeding tariff can affect the positioning of the solar panels


  • Solar will always produce power at the same time, all the solar panels will produce power at the same time across the entire city. So all panels will be producing at the same time. When it is dark they’re also all not producing power at the same time. This causes issues for the distribution networks where when all solar panels produce power at the same time it can affect the distribution network. So systems are in place to prevent you from installing too much power at certain times of the day.

Clear Sky Radiation received by a rooftop solar panel in Melbourne



Melbourne solar radiation (Global Horizontal Irradiance) - day to day variability

Shows the variability of solar output

Average solar radiation across Australia (Wm-2)

  • Irradiance higher/closer to the equator

  • Solar irradiance values differ horizontally because of clouds

    • when you are close to the ocean and you’ve got prevailing winds coming from the east in the tropics the airflow comes over the coast and you get lots of clouds forming as the air passes over the coast.

    • as the air moves across the continent, it dries out, and so we get much drier conditions on the west side of the continent and therefore less cloud cover, more solar irradiance.

    • If we move up into the tropics we are in the Intertropical Convergence zone, where we've got a lot of rising convection, a lot more cloud cover.

Global solar resource

Solar Photovoltaics (PV)

  • The only technology that converts sunshine directly to electricity

    • Silicon wafer

      • Polycrystalline silicon

        • easier to make, not as energy intensive but not as efficient

      • Monocrystalline silicon

        • recently improved dramatically

        • the efficiency gains from monocrystalline more than offset the additional costs of manufacturing monocrystalline silicion

      • Converts incoming photons into an electrical current with no moving parts

    • Thin film (CdTe,CIGS etc)

    • Organic PV

  • P-type silicon “doped” with boron or gallium

    • Deficit in electrons “holes”

  • N-type doped with phosphorous

    • Creates free electrons

  • Creates a dipole/electric field

    • One way flow of electrons


  • Silicon is a semi conductor so when a photon of radiation strikes the silicon it can excite an electron into the conduction band and causes current to flow.


Production or polysilicon

  • Pulverised MG-Si (metallurgical grade silicon) reacted with hydrochloric acid

    • Si + 3HCl → SiHCl3 + H2

  • SiHCl3 = Trichlorosilane (TCS) - gaseous

  • Purified in distillation towers

  • Chemical vapour deposition process

    • Pure polysilicon crystalises on a polysilicon ‘starter’

  • Recycles silicon tetra chloride

  • Energy cost of ~40 kWh/kg Si



  • Silicon does largely occur naturally so it needs to manufactured from silicon oxide.

  • To make metallurgical grade silicon we use a reduction reaction where we create carbon monoxide from fossil fuel and then use that to take the oxygen form the silicon oxide to produce pureish silicon

  • The silicon needed needs to be extremely pure so you take the metallurgical grade silicon and react it with hydrochloric acid which makes a gas trichloroethylene which can then be purified using distillation towers. We use a chemical vapour deposition process, and that gives us pure polycrystalline silicon that we can make the solar cells out of.

  • It is possible to recycle the silicon tetrachloride that we’ve reacted to in that purification process. But there’s still a significant energy cost here of about 40 kilowatt hours per kilogram silicon

Polycrystalline to monocrystalline silicon

  • But if you want to make monocrystalline silicon

  • And so we take some polysilicon and we melt it in a crucible in what’s known as a Czochralski apparatus

  • We melt this at very high temperatures, where you add doping materials at this point

  • We take a seed crystal of silicon and we load it into the melt, and we gradually lift the seed out and we rotate the crucible to draw out a single ingot of silicon

  • This crystal will have no discontinuities or impurities so it is more efficient.

  • Takes 30 kilowatt hours to produce

Assembly of a PV module

  • 6 × 10 Cells are sandwiched between two layers of encapsulant, usually EVA (ethylene vinyl acetate)

  • When heated to ~150 degrees, EVA ‘glues’ the cells and glass layers together

  • This is known as ‘laminating’

  • Backing material can be glass or polymer material


  • if the corners are cutoff then its a monocrystalline silicon


Deployment

Global solar PV installed capacity (GW)

PV deployment in Australia

New capacity in Australia per year (GW)

Other types of solar technology

  • Concentrating PV

    • Uses mirrors and/or lenses to focus lots of energy onto a high efficiency solar cell

  • Small scale hot water

  • Concentrating solar thermal

    • Linear Fresnel

    • Parabolic trough

    • tower

  • Solar chimney

Concentrating PV

  • Uses mirrors to focus sunlight onto highly efficient PV cell

  • More complex, uses solar tracking

  • Needs to be cooled

  • Solar systems (Mildura, mothballed,) and now RayGen

Small scale solar thermal

One of the simplest use of solar energy is to directly heat water

Solar collector

  • Flat plate (Common)

  • Evacuated tubes (better suited to cold climates)

Gravity or active pump

Linear Fresnel

  • Tracking mirrors focus sun on lines of working fluid (i.e. oil or water).

  • Uses boost steam production for fossil powered stations i.e. Liddell (NSW): 9 MW, Kogan Creek (QLD) : 44 MW, [neither operational], Puerto Errado (Spain): 31 MW

Parabolic trough

  • Similar concept to linear fresnel

    • sun focused onto tubes carrying working fluid (oil)

    • focal point narrower

      • therefore more efficient

      • and higher temperatures

    • Heat transfer to generate steam to drive turbine

    • But curved mirrors more expensive

  • Dominant technology type for concentrating solar thermal for electricity

  • Examples

    • SEGS (Mojave Desert, California) - 354 MW

    • Solnova (Sanlucar la Mayor, Spain) - 150 MW

    • Andasol (Guadix, Spain) - 100MW

Solar Tower

  • Large field of solar-tracking mirrors concentrate solar energy onto central tower

  • Working fluid (water/molten salt) transfers heat to stream to drive a turbine

  • Energy storage potential

Concentrating solar power (CSP)

  • Global deployment currently at around 8GW

  • Majority of the parabolic trough type

  • Majority in Spain and the USA

  • Largest is Ivanpah Solar Power Facility in California - 392 MW solar tower

Sundrop project - South Australia. Growing tomatoes in the desert

  • Heliostat array to produce fresh water

  • 39MWth (thermal)

    • Half into heating greenhouse

    • 30% for making water

    • 20% for electricity

  • Thermal energy storage tanks (water)

Compare solar with other technologies (IRENA)

Cost versus value

Summary

  • Solar radiation is in the short-wave frequency

  • Solar PV is an elegant technology, converting solar radiation directly into an electrical current

  • Solar PV prices have come down dramatically

  • Solar thermal captures the heat of the sun

    • Can be dispatchable => more valuable (but currently more expensive)

  • Small amounts of CSP to date, struggling to compete with PV and batteries