Concentrated Solar Power Park Design Project

This research and design paper was completed by Kaitlyn Burghardt as a component of the 3rd year chemical engineering class Energy & Environment at Western University. The goal of the project was to design a solar power generation park containing thermal energy storage, with the following design parameters and assumptions:

• The park can produce 71 MW of electricity
• The difference between the lowest and highest operating temperatures is 200ºc
• No energy losses in the electrical generator (η_elec.gen. = 0.99)

The design was required to have calculations for:

• Efficiencies for steam power generation and electrical power output
• Flow rate of molten salt
• Volume of the molten salt in the storage tank
• Required land area of the solar park

It is important to note that this project was designed to test students' overall understanding of solar power generation and to promote learning of thermal energy storage through research. Therefore, the calculation portion may not be entirely reflective of a realistic solar power generation park. This project received a grade of 96%.

Introduction[edit | edit source]

Sustainability is a design goal for researchers and designers across many industries, including processing, agriculture, engineering, and most importantly energy. Due to the increased impact of environmental movements in modern times, sustainable energy has become a key factor for professionals to consider. This low impact type of energy is defined as a form of energy that can be repeatedly used without being depleted or vanishing from the earth.^[1] Sustainable energy is also typically designed with harm reduction in mind to prevent greater environmental consequences to current and future environments.^[1] Renewable power generation, the root of sustainable energy, is sourced from natural processes with a faster generation rate than consumption rate.^[2] There are various types of renewable energy. These processes can directly or indirectly involve the sun, called solar-based renewable energy, or can come from the heat energy in the earth, called geo-thermal energy.^[2] Renewable energy can also be derived from interplanetary movement, called tidal energy.^[2]

Renewable energy, sometimes referred to as "new solar", is a main primary energy source on Earth and makes up 8.3% of primary energy distribution. Nuclear energy, which is comprised of fission, the splitting of atoms, and fusion, the combining of atoms, takes up 13.5% of the primary energy distribution on Earth. Fossil fuels, or "old solar" energy, are the remaining 78% of the primary energy distribution and consists of oil, coal, and natural gas. Because of harmful side effects of fossil fuels like greenhouse gas emissions, renewables are increasing in use. By 2050, renewable energy is expected to increase to 63% of the total primary energy distribution.^[3] Solar energy is desirable because it is free, as it comes from the Sun, but has an efficiency of around 20%.^[4] Research and design can be conducted to raise the efficiency of solar energy and make it a more viable solution.

Forms of Energy Produced from Direct Solar[edit | edit source]

Solar-based renewable energy can take many forms, including direct solar, wind, hydro, biomass, biofuels, wave, and salt gradients in water. Electromagnetic radiation is emitted by the sun in the form of light.^[5] That radiant energy can be directly turned into useful energy by photovoltaic technology (PV), concentrating solar-thermal power (CSP), or by photosynthesis.

Electrical[edit | edit source]

Photovoltaic technology is commonly exhibited in solar panels. The PV cells on the panels absorb energy from sun rays.^[5] The energy is converted to an electrical charge and causes electricity flow by an internal electrical field within the PV cell.^[5] A singular PV cell produces 1 to 2 watts of power, but when connected in an electrical grid system, can power larger needs like residential homes, commercial buildings and more.^[6] Commonly, solar panels need a solar battery to store energy.

Thermal[edit | edit source]

Another direct use of solar energy is solar heating, which can be performed by a concentrating solar power plant. Concentrating solar power systems use mirrors to reflect and concentrate sunlight onto receivers. Molten salt within the receiver then becomes heated and carries thermal energy.^[7] That thermal energy can flow through a system and be used to power an engine or rotate a turbine to produce electrical energy.^[7] The thermal energy can also be stored and used during times when sunlight is not available.^[5] Concentrating solar power systems are commonly used as power plants or for large projects, but an individual dish/engine system can also be used to produce 5 to 25 kilowatts of power.^[7]

Chemical[edit | edit source]

Plants convert solar energy to chemical energy in the process of photosynthesis. Put simply, photosynthesis is a fundamental process of life where plants take sunlight, carbon dioxide and water to create oxygen and glucose. Biofuel is a renewable process where biomass, plant matter, is burned to produce energy. Biodiesel is a common fuel which is comprised of alcohol, vegetable oil, and animal fat, and is used to power engines.^[8] Part of the energy found in plants initially comes from solar energy in photosynthesis.  

Examples of Energy Storage and Applications[edit | edit source]

Moreover, the amount of energy available to produce electricity in solar renewables varies based on location and time of day. An energy storage system where electrical energy from power systems can be converted into a storable form, and then converted back to usable electrical energy, is necessary to meet the fluctuating needs of the environment and society. Specifically in solar power, energy storage solutions are important because sunlight is only available during the day. To be used for operation at night, the energy needs to be stored. Examples of energy conversion and storage applications are discussed below.

Pumped Hydroelectric System (PHS)[edit | edit source]

One way to store electrical energy is through a pumped hydroelectric system. Electrical energy is converted to potential energy when electricity pumps water up to a reservoir. When that water is released, it moves a turbine to convert potential energy back to electrical energy and produce electricity.^[9] These systems are large-scale, as gravity is used as the converting energy.^[10] Newer facilities can have adjustable speeds to produce different power levels.^[10] They can also operate in a closed-loop system, which removes the need for a continuously flowing water source.^[10] PHS have a very high power rating of 3000 MW and efficiency of 70-85%, but are less energy dense.^[10]

Flywheel[edit | edit source]

Electrical energy can also be stored as kinetic energy in a flywheel. Electricity through a motor is used to rotate the flywheel where the energy is converted to kinetic.^[10] When electrical energy is again required, the motor can rotate the flywheel in the reverse direction and gain back the electrical energy.^[10] Flywheels are energy dense and have a high efficiency of 70-95%.^[10] The maximum power rating is around 20 MW.^[10]

Compressed Air System (CAS)[edit | edit source]

Additionally, electricity can be used to compress air into a high-pressure tank to be stored as pressure energy. The compressed air is then heated by natural gas and run through an expander turbine to generate electrical power.^[10] Compressed air systems have a maximum efficiency of 70% when the heat from the air pressure is recycled, but most often have an efficiency of 40-50%.^[10] These systems usually have a maximum power rating of 1000 MW, and a long lifetime of 40 years.^[10]

Batteries[edit | edit source]

Rechargeable batteries store electrical energy as electrochemical energy. In a lead-acid battery, a series of chemical reactions make the conversion when charged and discharged. More recently, lithium-ion batteries have increased in popularity because of their lightweight and energy dense design, compared to lead-acid.^[10] Lithium-ion batteries are used in hand-held devices to power electric vehicles.^[10] Most batteries have a maximum power rating of 100 MW, and are also energy efficient, with an efficiency of 80-95%.^[10]

Of the storage forms mentioned, batteries are the most efficient, followed by flywheel, pumped hydro, and finally compressed air. The different designs also vary in power rating, with pumped hydro able to produce the most, then compressed air, batteries, and flywheel. Energy storage forms can also be compared in terms of energy density, with lithium-ion batteries being the densest, then the flywheel, lead-acid batteries, compressed air, and least dense is pumped hydroelectricity. These different comparisons demonstrate a trend that larger, stationary systems like hydro and compressed air produce more energy, but at a lower efficiency, while smaller storage technologies such as batteries produce less power, but are more efficient.

Other forms of energy storage and transfer exist but were not discussed here. For example, the formation and breakage of chemical bonds involves energy transfer. Furthermore, different fluids can store thermal energy - measured by heat capacity - which can then be used to heat other fluids or materials, such as steam creation. Energy storage solutions are an important part of renewable energy processes and play an integral role in the practicality and usability of solar energy.

Literature Survey[edit | edit source]

The design of this solar park is a concentrating solar power plant (CSP) using thermal energy storage and a turbine. There are various CSP technologies, which are investigated below.

Parabolic trough collector (PTC)[edit | edit source]

This CSP technology is made up of long, curved mirrors that concentrate the sunlight onto a tube that stores a liquid, like molten salt, and runs parallel to the mirrors.^[11] The thermal energy from the liquid is converted into usable energy by producing steam that rotates a steam turbine.^[11] Storage integration is possible in PTC, and this system has a low installation cost compared to other CSP technologies.^[11] However, these plants require a large area.^[11] Also, because of low operating temperatures (usually a hot temperature of around 300ºc), they have a low thermodynamic efficiency of a maximum of 50% at a temperature of 800K.^[11]

Linear fresnel reflector (LFR)[edit | edit source]

Linear Fresnel Reflector systems concentrate sunlight onto a receiver tube by using multiple flat mirrors.^[11] Solar energy is transferred to thermal energy in the tubes, and this energy heats a liquid that then moves through a system to eventually rotate a turbine using steam.^[11] This technology is similar to the PTC design, and both systems are based on linear solar concentration.^[11] Like PTC, storage solutions can be integrated in LFR, and this system has a low installation cost.^[11] However, LFR occupies a large area, and has a very low thermodynamic efficiency of around 15% at operating temperatures of under 600K.^[11]

Solar power tower (SPT)[edit | edit source]

Solar power tower systems have large, flat, rectangular mirrors called heliostats that surround a tower.^[11] Light is reflected by the heliostats onto a receiver at the top of the tower, which holds molten salt.^[11] The molten salt is heated by the sunlight, and then flows through a system where it transfers thermal energy to boil water into steam. The steam turns a steam turbine that drives an electric generator.^[11] The spent steam turns back to water in a condenser and is pumped through the system to the boiler for re-heating.^[11] With SPT, the possibility for storage is very high and has a low cost; the molten salt can be stored for later use.^[11] This system also has a relatively high thermodynamic efficiency due to high operating temperatures of 60-70% at 1000K.^[11] Disadvantages of the SPT design are that a large area is needed, and that there is a high installation cost.^[11] The heliostats make up around 50% of the system cost, so they are important aspects to consider in designing a SPT.^[11]

Parabolic dish (PD)[edit | edit source]

A less common type of CSP technology is the parabolic dish. This system consists of a concave dish that focuses sunlight onto an elevated receiver in the middle of the dish.^[11] A piston-cylinder design then uses an engine located in the receiver to produce power.^[11] Disadvantages with this design is the difficulty of storage solution, and it has a high installation cost.^[11] Advantages include a high thermodynamic efficiency due to a high operating temperature of 65-80% at 1500K, and PD occupies a small area.^[11] Of the designs mentioned, this system is relatively new and produces the least amount of power.^[7]

Based on the characteristics of each technology, the Solar Power Tower system fits the requirements of the design project design due to its high efficiency and ability to produce and store energy. This design project system has a minimum operating temperature of 400K, so theoretical efficiency would be around 20%, as per the literature's thermal efficiency curve.^[11] Further, the working fluid (molten salt) stores thermal energy before it is used to convert water to steam to drive the turbine.

Design of Concentrated Solar Power Tower System[edit | edit source]

The design of a concentrated solar power system is complex. This technology has a system of storage tanks, a boiler, turbine, pump, condenser, electricity generator, receiver, and heliostats. To summarize the flow process, solar energy from the sun is reflected from heliostats onto a receiver that contains molten salt.^[11] That molten salt is held in a hot storage tank. Eventually, the liquid moves to a boiler where water is pumped to, and then the thermal energy from the molten salt heats the water which then turns into steam.^[11] The excess molten salt is transferred to a cold storage tank, and then to the power tower to again receive solar energy.^[11] The steam rotates a turbine which drives a generator, and usable energy is produced.^[11] Spent steam from the turbine is condensed and turned back to liquid water, where it is pumped back into the boiler for re-heating.^[11] This design portion focuses on choosing a location and molten salt. Calculations of efficiencies, heat, and flow rates are also displayed and discussed.

Choosing Location of Solar Park[edit | edit source]

The location of the concentrated solar power park had to meet several parameters due to feasibility and solar energy limitations. Because the amount of solar radiation that reaches the Earth's surface varies geographically, and that solar energy can only be extracted when there are no clouds, buildings or other obstructions in the light's path, certain locations are preferable. Coastal regions tend to be cloudy, mountainous regions have shadowed regions and an uneven landscape, and forest regions have trees, plant matter, and usually a high animal population. A flat, open field or desert, close to a populated area or other attraction that requires electricity, seemed the ideal location.

To further narrow down the location of the solar park, the rotation and shape of the Earth, and the tilt of it's axis had to be taken into account. Because of the spherical shape, sunrays hit the Earth's surface angles from 0º, near the horizon, and directly above at 90º.^[12] Extreme polar regions receive the least amount of sunlight because of the Earth's roundness.^[12] Vertical rays directly reach the surface of the Earth with a minimized loss of energy.^[12] The more slanted and closer the angle is to 0º, the rays become scattered and diffused as they travel longer through the atmosphere.^[12] Air molecules, clouds, pollution, forest fire smoke, and more particles can diffuse solar radiation and absorb or scatter sunlight.^[12] The tilt of the earth affects the amount of sunlight reaching the surface. In the northern hemisphere, more sunlight is seen in the summer than in winter, while in the southern hemisphere experiences the opposite. The equator, where the amount of sunlight stays relatively constant throughout the year, may seem like a strong contender for the location of a solar park. However, these regions tend to be very biodiverse and have plenty of flora and fauna, and thereby a solar park may promote ecological disruption. Taking all these factors into account, an ideal location for the solar park is a flat and open area close to a city, relatively close to the equator, that experiences direct beam solar radiation. The deserts near Joshua Tree National Park and Los Angeles in Southern California meet these parameters. This region has large, open areas with limited vegetation, high sun exposure, and is close to big cities like Los Angeles. The specific coordinates (34.357772º, -116.308829º), an area approximately 230 km from Los Angeles city centre, were chosen for data collection and calculations.

Choosing Type of Molten Salt[edit | edit source]

This solar park is designed to produce 71 MW of power. Sodium Nitrate (NaNO₃) was selected as the molten salt due to its high heat capacity, cost effectiveness, and successful use in other systems.^[13] Further, of the common molten salts, NaNO₃ has a relatively low melting point, which is preferable.^[13] Generally, molten salts are favorable to use in CSP systems because of their potential for heat storage.^[13]

Table 1: Thermal Properties of NaNO₃^[13]

Melting point (ºc)	Density (g/cm3)	Viscosity (mPa*s)	Specific heat capacity (J/molK)	Thermal conductivity (kW/molK)	Heat of fusion (kJ/mol)
306.5	1.900	3.038	131.8	5.66	15.50

Calculations[edit | edit source]

Table 2: Calculation Nomenclature

Symbol	Meaning	Units
η	Efficiency	%
E	Energy	W
Q̇	Heat flow rate	J/day
Q	Heat	J
ṁ	Mass flow rate	kg/hr
m	Mass	kg
V	Volume	m3
A	Area	m2
n	Number	unitless
DNI	Direct Normal Irradiation	MW/m2
GCR	Ground Coverage Ratio	%

Steam Power Generation Efficiency Calculation[edit | edit source]

To determine the input energy from the sun required to produce an output 71 MW, the efficiency for the steam power generation must be calculated according to the Carnot cycle. The low temperature was selected as 227ºc (400K), and the high temperature as 427ºc (600K), based on the design parameter of T_L-T_H = 200ºc, and that the melting point of NaNO₃ is 306.5ºc.^[13]

η_carnot = 1 - T_L/T_H

η_carnot = 1 - 400K/600K → η_carnot = η_{steam power generation} = 0.333 = 33.3%

Overall Plant Efficiency Calculation[edit | edit source]

The efficiency of the electrical generator is close to 100%, as there is an assumption of no losses (η_elec.gen. = 0.99). Using η_elec.gen, the required heat input from the boiler (same as the output from the sun) that eventually flows into the generator to produce 71 MW can be calculated:

η_elec.gen = E_out/E_{in, from boiler}

0.99 = 71 MW/E_{in, from boiler} → E_{in, from boiler} = 71.72 MW

Now, using η_carnot, the required power from the sun can now be calculated:

η_carnot = E_out/E_in = E_{in, from boiler}/E_{in, required}

0.333 = 71.72 MW/E_{in, required} → E_{in, required} = 215.16 MW

Since the energy required has been calculated, and that the output energy is known at 71 MW, the CSP plant's efficiency is:

η_overall = E_overall,out/E_{in, required} * 100%

η_overall = 71 MW/215.16 MW → η_overall = 32.99%

Mass Flow Rate of NaNO₃ Calculation[edit | edit source]

Now that the E_{in, required} has been calculated, the total heat can be found. Assuming a 12-hour day, the total heat to the receiver is:

Q̇ = E_{in, required} * 12 hr/day * 3600 s/hr

Q̇ = 215.16 MW * 12 hr/day * 3600 s/hr *10⁶ W/MW → Q̇ = 9.29*10¹² J/day

Now that Q̇ is known, the specific heat equation can be used to determine the mass flow rate of the molten salt. NaNO₃ thermal properties are as per Table 1:

c_p = 131.8 J/mol K * 1 mol/84.99 g → c_p = 1.552 J/g K

Q̇=ṁ*c_p*ΔT

9.29*10¹² J/day = ṁ * 1.552 J/g K * (600-400)K → ṁ = 2.99*10¹⁰ g/day = 1.25*10⁶ kg/hr

Minimum Storage Volume Calculation[edit | edit source]

It is now time to calculate the volume of the molten salt NaNO₃, in the storage tank for 12 hours of storage. The heat required to power the generator, the storage heat, would determine the amount of mass in the storage tank.  Q_storage can be calculated as:

Q_storage = E_{in, from boiler} * 12 hr * 3600 s/hr

Q_storage = 71.72 MW * 12 hr * 3600 s/hr * 10⁶ W/MW → Q_storage = 3.098*10¹² J

Mass in tank can be calculated as:

Q_storage = m*c_p*ΔT

1. 098*10¹² J = m * 1.552 J/g K * (600-400)K → m = 9.98*10⁹ g = 9.98*10⁶ kg

The volume of NaNO₃ in the storage tank can now be found from the calculated mass and the density per Table 1:

V = m/ρ

V = (9.98*10⁹ g) / (1.900 g/cm³) → V = 5.25*10⁹ cm³ = 5252.6 m³

Required Land Area of Solar Park Calculation[edit | edit source]

The required area and % of land usage must be calculated for the solar park system. The yearly direct normal irradiation (DNI) in the chosen specific coordinates (34.357772º, -116.308829º) is 2932.3  kWhr/m² per year (at the time of data extraction),^[14] which is equivalent to 0.000669 MW/m^2. Using this number and the required power of 215.16 MW, the total heliostat area can be calculated:

DNI = (2932.2 kWhr/m²year) * (1 year/365 days) * (1 day/12 hr) * (1 MW/1000 KW) = 0.000669 MW/m²

A_{total helio.} = E_{in, required}/DNI

A_{total helio.} = 215.16 MW/0.000669 MW/m² → A_{total helio.} = 321614.3 m²

The average mirror area of a single heliostat is 150 m^2[15]. So, the total number of heliostats is:

n_heliostat = A_{total helio.}/ A_{individual helio.}

n_heliostat = 321614.3 m²/150 m² → n_heliostat = 2144.1 = 2145 heliostats

The total number of heliostats was rounded up, as there cannot be a partial heliostat. The Ground Coverage Ration (GCR) is the ratio between the heliostat area and the total area, and the maximum ground coverage heliostats can take without touching another heliostat is 58%.^[16] The GCR also accounts for shadowing and the changing tilt of the heliostats, which is based on the position of the sun.^[16] The total area can then be found as:

GCR = A_{total helio.}/A_total

0.58 = 321614.3 m²/ A_total → A_total = 554507.4 m²

Table 3: Summary of Calculations

Steam power generation efficiency (ηcarnot)	Incoming energy from boiler (Ein, from boiler)	Required input energy (Ein, required)	Overall efficiency (ηoverall)	Total heat flow to receiver (Q̇)	Mass flow rate of NaNO3
0.333	71.72 MW	215.16 MW	32.99%	9.29*1012 J/day	1.25*106 kg/hr
Heat of NaNO3 in storage (Qstorage)	Mass of NaNO3 in storage (m)	Storage tank minimum volume (V)	Total heliostat area (Atotal helio.)	Total number of heliostats (nheliostat)	Total area of solar park (Atotal)
3.098*1012 J	9.98*106 kg	5252.6 m3	321614.3 m2	2145	554507.4 m2

Conclusion[edit | edit source]

The goal of this project was to design a concentrated solar power tower system capable of producing 71 MW, while meeting certain location and design parameters. Southern California was chosen at it has large, open deserts, and is close to cities and attractions. By using the Carnot cycle efficiencies and assuming an ideal electric generator, the required amounts of energy were calculated. Then, by using the specific heat equation and thermodynamic properties of the molten salt, the mass flow rate and volume of the storage tank was determined. The Direct Normal Irridation (DNI) and Ground Coverage Ratio (GCR) were used to calculate the total amount of land area required to produce 71 MW. This Concentrated Solar Power Park seems achievable, given that it produces such a large amount of power. This design produces 71 MW of power at an area of 554507.4 m², and requires 2145 heliostats. In comparison, the Ivanpah Solar Electric Generating System in Southeastern California produces 392 MW at 1.42 * 10⁷ m².^[17] Clearly, it takes a large area to produce megawatts worth of energy.

The overall efficiency is 32.99%, which exceeds the theoretical efficiency of 20%.^[11] However, there would be some inefficiencies and irreversibility in a real system, which would account for a lower real efficiency. For example, there would be high amounts of heat loss reflected from the heliostats and potential cloud and precipitation coverage. Further, the electrical generator would have losses, and would realistically not have an efficiency of 99%.

Moreover, the CSP technology's high ability to store solar energy makes it a great candidate for an energy source in the populous Southern California region, as energy can be stored for night usage. As a renewable energy source, the concentrating solar power system would reduce the need for fossil fuels and increase sustainability in energy.

References[edit | edit source]