Heat Transfer FluidsMicheal Araujo Mohammad Parhizi Sumeet Changla

Department of Mechanical and Aerospace EngineeringThe University of Texas at Arlington

Arlington, TX USA

Solar Thermal Energy Engineering ME5390

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Why Solar Energy is Important?[10]

• The traditional energy sources such as oil, natural gas etc, face a number of challenges including rising prices, and growing environmental concerns.

• Renewable energy sources such as solar, biomass, geothermal, hydroelectric and wind power generation have emerged as potential alternatives which address some of these concerns.

• Solar power generation has emerged as one of the most rapidly growing renewable sources of electricity.

• At any moment, about 1.74 x 1017 watts of energy from the sun strikes the earth which can be used to produce electricity.

• Photovoltaic and concentrated solar power plant are the available technologies to convert the sun’s energy to produce electricity.

• At present, 4 below technologies are developed for concentrated solar power plants:

• Parabolic trough power plant• Solar power tower system • Linear Fresnel technology• Dish sterling engine system

• Heat transfer fluids are used in most CSP technology branches.

• Their key function is to transfer the heat generated within the linear or the central receivers to the power conversion unit(s).

Types of Concentrated Solar Power Plants[9]

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Introduction to Heat Transfer Fluids [10]

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• Heat transfer fluid is a fluid which flows around or through a device to transfer the heat generated by the device to other devices which use or dissipate it.

• Heat transfer fluids play a crucial role in the efficient use of energy

• Heat transfer fluid suppliers have developed a variety of organic and water-based fluids to meet the operating needs of diverse applications.

• HTF have been used in many applications such as solar power, geothermal energy, natural gas compressors and secondary loop refrigeration.

• Due to the vast applications in industry, learning the properties of HTFs is crucial and a number of criteria must be examined before choosing a heat transfer fluid for a specific application.

Ideal Properties of Heat Transfer Fluids [3]

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• Potential to operate continuously at higher temperatures• Low viscosity, in order to reduce parasitic losses (at nominal temperatures

but also at low temperatures)• Possibility to be combined with heat storage systems• Low freezing point• Non-toxicity• Low cost• Specific gravity• High heat capacity• Sufficient pumpability• Thermal cycling tolerance• Sufficient long-term stability (low decomposition rate, low hydrogen

emission)• Chemical compatibility with common stainless steels or metals (corrosion)• Low environmental impacts in case of leakage

Types of Commonly Used Heat Transfer Fluids[1]

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• Air: Compressed air or gas is one of the heat transfer fluids which has been proposed to be used in a Brayton cycle in order to achieve higher thermal efficiency.

• Advantages: Since both the HTF and the working fluid is a gas, the maximum temperature

is no longer limited by the HTF or the working fluid (It will not freeze or boil). It is also non corrosive.

• Drawbacks: × Very low heat capacity× The low density of the gas/air× Since the gas or air is at high pressure, the level of complexity of design will

increase.

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• Water

• Advantages: Water is nontoxic and inexpensive high specific heat Because of very low viscosity, it's easy to pump

• Drawbacks: × Low boiling point and high freezing point.× It can be corrosive if PH is not maintained at a neutral level. × Water with a high mineral content (i.e., "hard" water) can cause mineral

deposits to form in collector tubing and system plumbing.

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• Hydrocarbon oils: The basic categories of hydrocarbon oils are synthetic hydrocarbons, paraffin hydrocarbons, and aromatic refined mineral oils.

• Synthetic hydrocarbons: Relatively nontoxic and require little

maintenance Paraffin hydrocarbons: Have a wider temperature range between

freezing and boiling points than water, but they are toxic and require a double-walled, closed-loop heat exchanger

Aromatic oils: The least viscous of the hydrocarbon oils.

• Advantages: These oils are relatively inexpensive. Low freezing point.

• Drawbacks: × Because of higher viscosity, they require more energy to pump.× Lower specific heat than water.

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• Molten salts: They are eutectic mixtures of inorganic salts with lower melting point than all the salts mixed.

• Molten salts are generally carbonate, chloride, nitrate ,and fluoride based.• The most widely used high temperature molten salt is known as solar salt.• It is a eutectic mixture of NaNO3:KNO3 in (60:40) by wt%.• It has a melting point of 222°C, and an operative temperature range of

between 260 and 567°C.

• Advantages: High boiling point Pollute less They are nonflammable. They are more abundant. They have lower vapor pressures. Offer cost savings due to smaller thermal tanks and piping.

• Drawbacks: × High freezing point.× Lower specific heat than water.

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• Glycol/Water mixtures: Glycol/water mixtures have a 50/50 or 60/40 glycol-to-water ratio.

• Ethylene and propylene glycol are "antifreezes." • These mixtures provide effective freeze protection as long as the proper

antifreeze concentration is maintained.

• Refrigerants/phase change fluids: These are commonly used as the heat transfer fluid in refrigerators, air conditioners, and heat pumps.

• They generally have a low boiling point and a high heat capacity. This enables a small amount of the refrigerant to transfer a large amount of heat very efficiently.

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• Silicones: • Examples of silicone is silicone oil .• It is an organic substance which is polymerized.

• Advantages:

Silicones have a very low freezing point, and a very high boiling point. They are noncorrosive and long-lasting.

• Drawbacks:

× Because silicones have a high viscosity and low heat capacities, they require more energy to pump.

× Silicones also leak easily, even through microscopic holes in a solar loop.

Ongoing R&D on Heat Transfer Fluids[2],[3]

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• The main on-going research activities are:

• Direct steam generation is being developed on demonstration level by Schott, Siemens, Solar Millennium, Iberdrola together with research institutes.

• Development of several new HTF formulations with improved characteristics:

Improved synthetic oils, with a better heat transfer coefficient by Dow and Solutia

Molten salt systems (sodium nitrate / potassium nit rate) by Schott and Archimedes Solar (linear receivers), by Sener (central receiver) and by Coastal Chemicals and Halotechnics in the USA.

Alternative inorganic materials with lower freezing points than molten salt are being developed by for instance. Dow Corning.

Ongoing R&D on Heat Transfer Fluids[2], [3]

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• R&D organizations such as CIEMAT, CNRS, DLR, ENEA, Fraunhofer ISE, CEAINES in Europe, NREL and Sandia in the USA and .CSIRO in Australia are involved in additional topics such as:

Nanoparticle enhanced HTF’s (oil and molten salts) with the objective to improve heat capacity.

Encapsulated PCM nanoparticles Metallic PCMs compatible with molten salt HTFs

• Facilities capable of performing :

Fluid property measurements Interface studies: HTF / absorber, storage tank, PCM Long-term field testing of fluid performance and stability

Ongoing R&D on Heat Transfer Fluids[2], [3]

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• Challenges • Improved HTF formulations with :

Increased thermal stability to temperatures over 400°C for oil-based HTFs (avoiding hydrogenation) or to over 500°C for molten salts.

Melting points below 100°C. This can be improved by means of alternative molten salt formulations, for example by varying the nitrate/nitrite ratio and the concentration of components such as lithium, sodium, calcium and potassium.

• Development of :

Models to predict the main properties such as heat capacity, melting point, heat of fusion, and density.

characterization methods to check as quickly as possible these properties and their ageing rates.

HTFs Applications in Different Solar Power Systems[3]

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• Fresnel linear technology: • Fresnel systems can use synthetic oil. However, as the absorber design

results in less flexible hoses and moving parts, it seems a good candidate for adoption of water/steam as HTF and for withstanding the high pressures of superheated steam (100bars, 500°C).

Power Towers [3], [11]

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• Working fluid can be water, molten salt, liquid sodium or air

• First solar power towers used water as a working fluid.

• However, nowadays in USA solar power towers use as a working fluid mostly molten nitrate salt that is not flammable, is non-toxic, and is better as storage of heat than water.

• In Europe solar power towers use air as a working fluid.

• Molten salts were used in the range of 500-600°C in the first R&D projects (Themis in the 80s, Solar Two in the 90s).

• Current developments focus mainly on superheated steam at 550°C and still on molten salt technologies (Gemasolar).

• But gases such as air, hydrogen, helium and CO, as well as liquid metals, are also candidates for investigation.

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Example of Power Tower with Molten Salt as HTF

Power Tower [3], [11]

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• First solar power towers used water as a working fluid.

• However, nowadays in USA solar power towers use as a working fluid mostly molten nitrate salt that is not flammable, is non-toxic, and is better as storage of heat than water.

• In Europe solar power towers use air as a working fluid.

• Molten salts were used in the range of 500-600°C in the first R&D projects (Themis in the 80s, Solar Two in the 90s).

• Current developments focus mainly on superheated steam at 550°C and still on molten salt technologies (Gemasolar).

• But gases such as air, hydrogen, helium and CO, as well as liquid metals, are also candidates for investigation.

Parabolic Troughs [3], [11]

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• The heat transfer fluid is usually water or oil, where oil is generally preferred due to its higher boiling point.

• Highest level of development is synthetic oil with a maximum temperature of about 400°C. Higher operating temperatures result in formation of oil decomposition products which need to be removed more frequently.

• Developments mostly target improved synthetic oil, molten salts or steam, which allow higher temperatures.

• Molten salts have the disadvantage of high freezing points, whereas steam requires the development of receivers and flexible hoses and joints capable of withstanding higher pressures.

• The preferred boiling system implements direct steam generation (DSG), where water is the heat transfer fluid.

Parabolic Troughs [3], [11]

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• In a parabolic mirror solar power plant (trough design), the heat transfer fluid (HTF) is heated to elevated temperatures using the energy from the sun.

  • The thermal energy in the HTF is used to convert water to superheated

steam in a series of heat exchangers namely the pre-heater, steam generator and the super-heater.

  • The thermal energy in the HTF is also used to heat the steam exiting the

high-pressure steam turbine in a re heater. 

• The pre-heater, steam generator, super-heater and the re-heater are commonly referred to as the solar power plant heat exchangers.

• Direct steam allows higher operational temperatures than possible with either molten salt or thermal oil.

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  • The feasibility of the direct steam generation (DSG) process in parabolic-

trough collectors (PTCs) has already been proven in the direct solar steam (DISS) project under real solar condition, at pressures up to 100 bar and temperatures up to 400 C, with more than 10,000 h of operation. To explain how heat transfer fluid flows, we chose the DISS solar field test facility located at the Plata-forma Solar de Almeria (PSA) in Spain.  

Direct Steam Generation [12], [13]

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• A key area of investigation in parabolic-trough collectors is the thermal stress in the absorber pipes.

• And more important when the heat transfer fluid is water, the effect can be compounded if the two-phase flow inside the absorber pipe is stratified, meaning that the liquid water flows at the bottom of the pipe and the steam flow at the top.

• It is important to mention that for DSG systems, the temperature difference registered between the hottest and the coldest points over the external wall of the pipe will increase if feed flow is too high. This is a result of non-constant heat transference from the receiver to the HTF, and can potentially affect the quality of produced steam.

Direct Steam Generation’s Concerns [12], [13]

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  Challenges:

• While it is possible to build a regular steam cycle running at supercritical pressures, it is a technical challenge to create receiver equipment that can handle high pressures.

• Another limiting factor is the trade-off between receiver tube performance and maximum operating pressure. Higher pressures means that the receiver tubes need to be made of thicker walled tube. The thicker walls impede the heat transfer performance of the receiver tubes.

• Suitable thermal energy storage.

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  Advantages and drawbacks of DSG: No heat exchangers High temperatures High efficiency Non-toxic fluid× Two phase flow× Higher control effort× Expensive thermal storage× Higher temperature gradients

• Advantages and drawbacks of oil: Commercially applied One-phase flow Easily scalable× Heat exchanger batteries× Temperature less than 400 C× Efficiency limit reached× Hazardous to environment

Comparison between DSG and Oil [12], [13]

Heat Exchangers[3]

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• The oil-to-salt heat exchangers must be designed with very small approach temperatures, in the range of 3–10 vC.

• In addition, the vapor pressure of the heat transport fluid is approximately 10 bar at the normal collector field outlet temperature of 390c

• Adding the pressure loss in the heat exchangers and the piping system, the pressure at the inlet of the heat exchanger is maintained at a nominal value of 20 bar.

• In contrast, the vapor pressure of the nitrate salt is very low (<1 Pa), and the pressure of the salt in the heat exchangers only that necessary to circulate the salt, or perhaps 5 bar.

• The most economical heat exchanger which provides these features is a conventional shell and tube design. The high-pressure heat transport fluid is placed on the tube side, and the nitrate salt is placed on the shell side. The tubes are rolled and seal welded to the tube sheet to improve the reliability of the exchanger.

Piping System[4]

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 • An absorption pipe is four meters long and composed of a multi-layered

stainless-steel pipe, which has an absorption level of 95%.

• It radiates 14% of its heat at temperatures of around 400 degrees Celsius.

• The steel pipe is surrounded by a vacuum-isolated concentric borosilicate glass cladding tube with anti-reflexcoating, which allows for over 96% penetration solar radiation.

• Approximately 22,500 absorption pipes are used in each power plant. The challenge when it comes to the longevity of the tubes is the interface between the steel and cladding tubes, which are sealed to form an intermediate vacuum.

• The different thermal expansion of the steel tube and the glass cladding is compensated by a metal bellows.

• A heat transfer medium circulates in the pipes, theso-called heat transfer fluid (HTF).

Nanofluids as heat transfer fluids[5]

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Definition:

• Suspensions of solid submicron- and nanometer-sized particles in various fluids (also called nanofluids)

• what are nanoparticles:a sub-classification of ultrafine particle with lengths in two or three dimensions greater than 0.001 micrometer (1 nanometer) and smaller than about 0.1 micrometer (100 nanometers)

• Nanoparticles has high surface to volume ratio which provides high surface area available for the reaction.

Advantages[6]

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• Advantages of nanofluids:

• High heat capacity

• High thermal conductivity

• High specific surface area and therefore more heat transfer surface between particles and fluids.

• Reduced pumping power as compared to pure liquid to achieve equivalent heat transfer intensification.

• Reduced particle clogging as compared to conventional slurries, thus promoting system miniaturization.

• Adjustable properties, including thermal conductivity and surface wettability, by varying particle concentrations to suit

Advantages in CSP[7]

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• Advantages of nanofluids as HTFs in CSP:

• Cost can go down significantly

• It is estimated that the enhancement in the specific heat of the alkali salt eutectic by 30% can enable a reduction in the cost of solar thermal power by more than 15%

• Disadvantages of nanofluids as HTFs in CSP:

• Cost can go down significantly

• High viscosity

• High freezing point

Application[8]

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• Application of nanofluids in general:

• Cooling of electronics

• Engine cooling/vehicle thermal management

• Cooling of diesel electric generator

• Solar water heating

• Cooling and heating in buildings

• Application in transformer

• Application in nuclear reactor

Reference]

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1. Johannes P. Kotzé, Theodor W. von Backström, Paul J.Erens., "NaK as A Primary Heat Transfer Fluid in Thermal Solar Power Installations", Southern African Solar Energy Conference, May 2012.

2. David Barlev, Ruxandra Vidu, Pieter Stroeve, "Innovation in Concentrated Solar Power", Journal of Solar Energy Materials & Solar Cells, Vol.95, PP.2703–2725, October 2011.

3. Peter Heller, Andreas Häberle, Philippe Malbranche, Olivier Mal, Luisa F. Cabeza., "Strategic Energy Technology Plan, Scientific Assessment in support of the Materials Roadmap Enabling Low Carbon Energy Technologies: Concentrating Solar Power Technology", Institute for Energy and Transport, Luxembourg: Publications Office of the European Union, 2011

4. D. Kearney, B. Kelly, U. Herrmann, R. Cable, J. Pacheco R. Mahoney, H. Price, D. Blake, P. Nava, N. Potrovitza., "Engineering Aspects of a Molten Salt Heat Transfer Fluid in a Trough Solar Field", Journal of Energy, Vol 29, PP.861–870, April-May 2004.

5. Solar Mellennium AG, "The Parabolic Trough Power Plants Andasol 1 to 3, The Largest Solar Power Plants in The World", Technology premiere in Europe.

Reference]

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6. A. Alagarasi., "Introduction to Nanomaterials", pp.76. 7. Application of Nanofluids in Heat Transfer -P. Sivashanmugam8. D. Malik., "Evaluation of Composite Alumina Nanoparticle and Nitrate Eutectic

Materials for Use in Concentrating Solar Power Plants (Thesis)", Texas A&M University, College Station, TX, 2010

9. R. Saidur, K.Y. Leong, H.A. Mohammad., "A Review on Applications and Challenges of Nanofluids", Renewable and Sustainable Energy ReviewsInternational Journal of Heat and Mass Transfer, 57 582–594, 2013.

10. Neil Canter., "Heat Transfer Fluids: Selection, Maintenance and New Applications",2009

11. Tomislv M.Pavlovic, Ivana S.Radonjic, Dragana D Milosavljevic, Lana S.Pantic., "A Review of Concentrating Solar Power Plants in The World and Their Potential Use in Serbia", Journal of Renewable and Sustainable Energy,16, 3891-3902, 2012.

12. David H. Lobón, Emilio Baglietto, Loreto Valenzuela, Eduardo Zarza. "Modeling Direct Steam Generation in Solar Collectors with Multiphase CFD", Journal of Applied Energy, 1338-1348, 2014.

13. Fabian Feldhoff. , "Direct Steam Generation (CSP) Technology Overview", SFERA Summer School, Almeria Spain, June 2012.

Thank You!