Design of Renewable Energy Systems

• Solar PV converts photon energy directly into electrical energy via the photovoltaic effect in a semiconductor p–n junction

• Key advantages: modularity, low marginal cost, rapid scalability

• ~90% module price decline since 2010

• Dominant form of new electricity generation capacity globally

• Utility-scale and distributed deployment

• CSP uses optical concentration to convert sunlight into high-temperature heat, which drives a thermodynamic power cycle (typically Rankine)

• Key strength: integrated thermal energy storage (typically molten salt)

• Provides dispatchable renewable power

• Wind turbines convert the kinetic energy of moving air into mechanical rotation, which drives an electrical generator (aerodynamic lift-based energy conversion)

• One of the lowest-cost sources of new bulk electricity

• Large multi-MW turbines (10–15+ MW offshore)

• Onshore mature; offshore rapidly expanding

• Key challenges: variability, transmission integration, siting constraints

• Increasing role in high-renewable power systems

Hydropower converts the potential and kinetic energy of water at elevation into mechanical shaft power via a turbine, which drives a generator.

• Largest source of renewable electricity globally (by annual generation)

• Mature, high-efficiency technology (>90% turbine efficiency typical)

• Provides grid stability and dispatchability

• Pumped hydro is dominant form of large-scale energy storage

• Constraints: geography, environmental impact, social license

1. Intermittency & Dispatchability

• Wind, sun, and other renewables are intermittent — they are not available at all times when we will need them

• Dispatchability is the capability to increase/decrease supplied energy in response to changes in demand

1. Different Demands

• Electricity demand isn’t the only sector needing to be decarbonized

• Some are very challenging (transport - ships, industrial - steel production needing high & high power furnaces)

1. Material constraints

• Availability of key minerals and rare earth elements at sufficient scale (e.g. Lithium, copper)

• Finding suitable high-temperature materials for some promising technologies for industrial process heat (e.g. concentrating solar)

1. Electricity grid updating

• Large scale renewable energy systems are often in remote areas

• Large scale construction of new lines will be needed

1. Costs (still!)

   Cost (LCOE) is now competitive with fossil fuels for all major renewables, but…

• Many investors perceive renewables as higher risk (and charge higher interest)

• Large capital cost

• LCOE does not necessarily account for every cost such as new transmission infrastructure

Elictricity demand over daily hours - due to high output of electricity of rewnables during the day the demand lowers at day but remains high at night. (related to challenge 1 of renewables)

—> The trend is that negative demands in the middle of the day —> This results in curtailed (wasted) energy in the middle of the day.

Capital Expenditure (CAPEX): upfront costs incurred to develop, build, or acquire (an energy system).

• Usually one-time, high-cost investment.

• Covers physical assets and infrastructure. Typically depreciated over time.

Operating Expenditure (OPEX): recurring costs associated with the daily operation and maintenance of an energy system.

• Includes fuel, maintenance, labor, and administrative costs

CAPEX is often high for renewables, but OPEX is typically low

• Net Present Value (NPV): Measures profitability over time by discounting future cash flows

• Internal Rate of Return (IRR): The discount rate at which an investment breaks even

• Levelized Cost of Electricity (LCOE): Average cost of generating electricity over lifetime

Appendix:

• defines the system of interest, everything else is “surroundings”

• Phase is a condition of matter (e.g. solid, liquid gas)

• The kinetic energy of the molecules (related to chances in temperature)

• The internal energy associated with a change in phase (e.g. boiling)

• The internal energy associated with the atomic bonds in a molecule

• The internal energy associated with the bonds wining the nucleus of the atom itself

• Conservation of energy principle: During an interaction, energy can change from one form to another but the total amount of energy remains constant. Energy cannot be created or destroyed. •

• The first law of thermodynamics: An expression of the conservation of energy principle.

• The second law of thermodynamics: It asserts that energy has quality as well as quantity, and actual processes occur in the direction of decreasing quality of energy.

• The second law has fundamental implications, but its study is outside our scope

eta_th = 1 - Q_L/Q_H

• Ideal process of heat cycle

• eta_carnot = 1-T_L/T_H

• eta_th < eta_carnot

The practical variant of the Carnot cycle for steam/water

Conduction through a solid or a steady liquid, convection from a surface to a moving liquid, radiation

Radiation is energy transported by electromagnetic waves

lambda = c/v

v = frequency

• Diffuse: direction independent

• Grey: wavelength independent

• Opaque: no transmission through surface

an ideal surface that emits radiation at the maximum rate

• Incident radiation on a surface can be absorbed, transmitted, or reflected

• For an opaque body (right) it can only be absorbed or reflected

Radiation absorption is characterized by 𝑄°_𝑎𝑏𝑠𝑜𝑟𝑏𝑒𝑑=𝛼 𝑄°_𝑖𝑛𝑐𝑖𝑑𝑒𝑛𝑡

where 0≤𝛼≤1 is the absorptivity

• Kirchoff’s Law: under certain conditions, the emissivity and absorptivity are equal

• —> alpha = epsilon

The view factor 𝐹𝑖𝑗 describes the fraction of radiation leaving surface 𝑖 that is intercepted by surface 𝑗

• Laminar flow: a constant (in time) velocity under steady conditions

• Turbulent flow: significant randomness — even if the predominant flow direction is still along the pipe

• Transition characterized by the Reynolds Number, 𝑅𝑒=𝜌𝑉𝐿/𝜇 , where 𝐿 is a characteristic length

• The fluid’s interaction with the surface results in a velocity boundary layer

• Within this boundary layer, viscous forces are significant (so Bernoulli isn’t applicable)

• Along the direction of flow, the boundary layer grows until steady conditions are achieved

The angle of attack alters the lift drag balance — when it is too high, the flow separates, and the lift drops dramatically.

Yes, it depends on the highest on lowest temperatures

—> eta_carnot = 1 - T_L/T_H

Radiation is energy transported by electromagnetic waves, wavelength 𝜆 and frequency 𝜈, which obey:

• lambda = c / v

The solid angle ω is the three-dimensional analogue of a planar angle, measuring how large an area dA on a sphere's surface appears from its center, defined as dω = dA/r² in steradians.

In terms of the spherical coordinates for the emitting surface on the right

𝑑𝜔=sin𝜃 𝑑𝜃 𝑑phi

addition: In spherical coordinates, θ (theta) is the polar angle measured from the surface normal n — so θ = 0° means you're looking straight up from the surface, and θ = 90° means you're looking along the surface. φ (phi) is the azimuthal angle, measured horizontally around the normal in the base plane, essentially describing the compass direction of the emitted radiation.

Diffuse radiation is when the radiance shows no directional dependence 𝐿(𝜆, 𝜃,𝜙) → 𝐿(𝜆)

Irradiance is the rate at which radiation is incident on a surface from all directions

Black body

• Black bodies are diffuse emitters

Spectral radiance (a.k.a radiation intensity) of a black body is dependent only on temperature

How much a body can emit in relation to the theoretical maximum (black body emissivity)

0 < epsilon < 1

Absorptivity α describes how well a surface absorbs incident radiation (α = 1 = blackbody again)

And the elegant connection between them is Kirchhoff's Law:

ε_λ​=α_λ​

diffuse, specular, or both

Diffuse reflection: Incident radiation is scattered equally in all directions, independent of the incoming angle — like light hitting a chalk surface.

Specular reflection: Incident radiation is reflected in one specific direction where the angle of reflection equals the angle of incidence — like light hitting a mirror.

𝜀_𝜆 = 𝛼_𝜆 if either

1. The surface is diffuse

2. The irradiance is diffuse •

𝜀 =𝛼 if, in addition to 1 or 2 above, either

3. Surface is grey (𝜀𝜆, 𝛼𝜆 independent of 𝜆)

4. The irradiance corresponds to emission from a black body

Atmospheric attenuation is the reduction in radiation intensity as it travels through the atmosphere, caused by absorption and scattering by gases (like CO₂, H₂O) and particles (like dust or aerosols) — which is why the sun appears less intense at sunrise/sunset when light travels through a longer atmospheric path compared to midday.

• Atmospheric attenuation depends on the length of the path through the atmosphere

• Air Mass is the length of the path through the atmosphere, normalized by the shortest possible path (when the sun is directly overhead)

Solar zenith angle, 𝜽𝒛 (degrees). The angle between the vertical and the line to the sun, that is, the angle of incidence of beam radiation on a horizontal surface.

Solar azimuth angle, 𝜸𝒔 (degrees). The angular displacement from the projection of the beam radiation onto the horizontal plane. Usually, zero is due south and displacements east (west) of south are negative (positive)*.

Solar altitude angle, 𝜶𝒔 (degrees). The angle between the horizontal plane and the line to the sun, that is, the complement of the solar zenith: 𝛼𝑠=90∘−𝜃𝑧.

Longitude, 𝑳. Angular location west of the prime meridian. We use the convention that 0≤𝐿≤360 increasing westward. Brisbane longitude of 153.0260 E in this convention is 𝐿𝐵𝑟𝑖𝑠𝑏𝑎𝑛𝑒=360−153.0260∘=206.974∘

Latitude, 𝝓. The angular location north or south of the equator. Positive is north, −90∘≤𝜙≤90∘

Hour Angle, 𝝎. The angular displacement of the sun east or west of the local meridian due to the rotation of the earth (at 15∘ per hour). Morning is negative, afternoon is positive.

𝜔 =15⋅ (hours before solar noon)

=15* (𝑡𝑠𝑜𝑙𝑎𝑟 − 12)

—t_solar is solar time — a time system where noon is always exactly when the sun is at its highest point (crossing your local meridian) —

Declination angle, 𝜹. is angular position of the sun at solar noon (i.e. when the sun is on the local meridian) with respect to the plane of the equator. North positive, � � =23.45∘ at northern summer solstice.

Pyrheliometers Measure solar radiation near normal incidence (DNI). This is the irradiance experienced by a plane normal to the direction of the solar radiation propagation.

Pyranometers Measure total hemispherical solar radiation (beam + diffuse), usually on a horizontal surface (GHI).

• Direct (beam) — coming directly from the sun without any scattering

• Circumsolar diffuse — forward scattered (via Mie scattering)

• Isotropic (sky) diffuse — coming equally from all parts of the sky dome

• Horizon diffuse — due to the horizon brightening effect (e.g. due to ground reflected irradiance being re-scattered toward the collector)

• Ground reflections (often assumed to be diffuse)

Wind energy is a byproduct of the sun. The sun’s uneven heating of the atmosphere, the Earth’s irregular surfaces (mountains and valleys), and the planet's revolution around the sun and its own axis all combine to create wind.

Global phenomena known as Synoptic: related to pressure difference (temperature variations) across the globe, hence more constant throughout the year

Local phenomena known as Diuturnal: related to a local temperature variations, can thus change daily or seasonally (e.g., sea breeze and land breeze)

wind speed - determines the amount of kinetic energy which can be extracted from the wind

Hours per year over wind speed —> probability distribution

In particular the Weibull distribution:

PDF (probability density function)

CDF (cumulative distribution function)

Represents the number of annual hours for which the wind speed equals or exceeds a particular value.

Provides an idea about the nature of the wind regime at each site.

The total area under the curve is a measure of the average wind speed.

A flatter curve indicates more constant wind speeds, while a steeper curve indicates more variable wind speeds

It can be used to predict the power duration curve

No, increases the further away it is from the ground until it reaches the free-stream speed

at sea that is reached faster than at land

The wind power density expresses how much power can be collected per m^2 of surface

WPD = P_w/A

It's the theoretical maximum efficiency of any wind turbine — no matter how well designed, you can never extract more than 59.3% of the wind's kinetic energy

Cut-in speed is the minimum wind speed at which useful power can be generated. (usually around 3-4 m/s)

Rated speed is the wind speed that delivers the rated power, usually the maximum power.

Cut-out speed is the maximum wind speed at which the wind turbine is designed to produce power. (usually around 25 m/s). At wind speeds greater than the cut-out speed, the turbine blades are stopped by some type of braking mechanism to avoid damage and for saf

(Tip-speed ratio (λ) is the ratio between the tangential velocity at blade tip (ωR) and the absolute undisturbed wind speed (v))

It represents the number of annual hours for which the generated wind power equals or exceeds a particular value.

It is derived from the wind speed duration curve

The total area under the curve is the annual energy production for a given turbine in a chosen site.

Orientation of rotation axis:

• HORIZONTAL AXIS WIND TURBINES (HAWT)

• The majority of commercial wind turbines

• Yaw systems

• VERTICAL AXIS WIND TURBINES (VAWT)

• Lower aerodynamic efficiency than HAWT

• No yaw system required

Torque generating mechanism:

• LIFT

• DRAG

The goal is to maintain the optimal angle of attack at every radial station so that each section operates at its best C_L/C_D ratio — maximising aerodynamic efficiency across the entire span.

1. Identify candidate regions Wind resource atlas + available data to find high-average-speed areas in the region of interest

2. Select candidate sites Topography, ecology, and public acceptance; detailed terrain analysis if needed

3. Preliminary site evaluation Rank sites by economic potential; assess environmental impact and operational constraints

4. Final site evaluation Comprehensive measurement: wind speed, shear, turbulence, and prevailing direction

5. Micrositing Determine exact turbine locations; model wake interactions to maximise energy capture

25%-40%

Offshore:

• Higher and more stable wind speeds

• • More uniform wind speed

• Reduced blades stresses due to uniform wind

• No ground occupation

• Higher structural issues

• More expensive power wires

• Higher complexity and installation costs

Onshore:

• Lower costs of installations and maintenance

• Mature technology

• More unsteady wind and more significant variation with height

• Noise and visual impact

• Land occupation

• Onshore^^

higher LCOE

It offers inepensive storage & dispatachability (Molten salt)

well understood power cylce (steam turbine)

efficient supply of industrial heat

The concentration ratio 𝐶 is the ratio of the aperture area 𝐴𝑎 to the receiver area 𝐴𝑟:

C=A_a/A_r

C_max,3D= 46,165

C_max,2D= 215

Optical efficiency describes how well the concentrator focuses radiation onto the receiver surface.

Receiver thermal efficiency describes how well the receiver transfers the incident power to the heat transfer fluid (HTF)

Carnot efficiency (heat engine efficiency)

Higher concentration ratio → curves shift right → receiver stays efficient at much higher temperatures.

Why? More concentrated radiation means more power input per unit area, so radiation losses (which scale with T⁴) become relatively smaller compared to the useful heat collected.

Sun shape. See previous section.

Specularity errors. Degree to which reflected rays obey reflection law (microscopic)

Surface slope errors. Distortions of the local surface normal vector w.r.t the ideal.

Shape error. Reflective surface is often constructed of individual facets, which may be oriented incorrectly.

Tracking error. Incorrect orientation of the trough by the tracking system.

Receiver deviation error. Receiver is not precisely on focus line.

End losses. Spillage of radiation at the end of the collector.

The major losses are

1. Radiative losses: tube → glass envelope → surroundings.

2. Convective losses: tube → glass envelope → surroundings. Tube → envelope convection only significant if there is a gas in the gap (e.g. via a leak, or 𝐻2 infiltration).

3. Conductive losses through the support brackets.

Cosine losses:

The effective reflecting area of a heliostat depends on the angle between the heliostat surface normal and the incoming sun ray. Even if the heliostat is perfectly reflective, if it's tilted at an angle θ to the sun, the projected area catching sunlight is reduced:

A_eff​=A_heliostat​⋅cosθ

Higher receiver temperature → higher cycle efficiency, but higher losses (right)

Increasing the concentration ratio permits higher temperatures (and lessens the impact of receiver losses (smaller receiver)

Low power outputs and complex piping systems

Far from equator —> Polar field

close to equator —> Surrounded field

CR usually in the range of 500-1000

• denote cosine losses

• shading from other heliostats & tower

• blocking of reflected rays

• air attenuation:

  • Lenght of reflected rays

  • Athmospheric condition (dust, fog visibility)

• reflectance of the heliostats degrades over time (soiling e.g. dust (recoverable) or corrosion (non-recoverable))

• spill: reflected radiation that misses the receiver - to catch more rays you can increase the receiver surface but that increases the heat losses (radiative (choose coating with low emissivity), convective (high e.g. wind) & conductive (low))

Cloud passages involve a change in total solar power and heat flux distribution

highly non uniform heat flux can lead to creep or fatigue of the receiver material

• High evaporation temperature

• low freezing temperature

• Thermal stability

• High heat capacity

• High heat conductivity

• Low viscosity

• Low investment cost

• Availability

• Environmental compatibility

• low inflammability

• low explosivity

• Synthetic oils

  • T_max = 400°C & state of art for Parabolic Trough

• Solar salts

  • Tmax = 600°C & Tmin = 250°C state of the art for solar towers

• Steam/water

  • high temperatures & high pressures make storage challenging

Direct cycle:

The power block working fluid coincides with the SF heat transfer fluid.

• No heat exchanger between HTF and working fluid

• The HTF maximum temperature is the power cycle maximum temperature

Indirect cycle: A different fluid is used for the power block and in the solar field + working fluid/HTF can be optimised

• CSP

• —> Power Tower

• —> Indirect cycle layout

The addition of thermal sotrage implies oversizing the solar field

—> To both produce and store under nominal conditions

Oversizing is quantified by the Solar Multiple (SM)

With the thermocline configuration, only one vessel has the same energy content as two tanks (potential economic savings)

Moreover, the tank can be filled with cheap high thermal capacity material, hence further reducing the costs of the storage

𝑉𝑠 − 𝑣𝑅 − 𝑣𝐿 − 𝑣𝑐 = 0

&

v = iR

Resistance R is a special case of impedance — it only applies to resistors and is a real, frequency-independent value. It simply opposes current flow and dissipates energy as heat.

Impedance Z is the general concept that describes how any component opposes current flow in an AC circuit. It can be complex-valued and frequency-dependent.

Z=R+jX

where X is the reactance — the imaginary part contributed by inductors and capacitors.

series: Zs = Z1+Z2

parallel: Zp = Z1Z2/(Z1+Z2)

v(t) = Vm cos(omega t)

i(t) = Im cos(omega t + theta)

same frequency but shifted by theta

p_avg = I^2R = IV

note: the capital letters describe the RMS quantities

Capacitor: current leads by 90°

Inductor: voltage leads by 90°

A resistor just dissipates the energy, a capacitor saves it R~ Xc = 1/(omega*C)

—> a high frequency omega leads to small resistances for a capacitor

Real power P (Watts) — Power that actually does useful work (heat, light, motion). P = VI cos φ

Reactive power Q (VAR) — Power that oscillates back and forth between source and inductor/capacitor, does no useful work but still causes current to flow through the lines. Q = VI sin φ

Apparent power S (VA) — What the utility actually has to supply — the total combination of P and Q. S = VI = √(P² + Q²)

Power factor PF — Ratio of useful to total power, describes how well aligned voltage and current are. PF = cos φ = P/S. PF = 1 is ideal, PF < 1 means the utility supplies more than the customer uses.

DC-DC (Chopper): Control of DC voltage magnitude

AC-DC (Rectifier): Control of DC voltage, AC current

DC-AC (Inverter): Control of AC voltage magnitude and frequency

AC-AC (Cycloconverter): Control of AC voltage magnitude and frequency

Create DC from AC

Single phase - full wave :

Three phase - half and full wave: