The links below provide an outline of the material for this lesson. Be sure to carefully read through the entire lesson befor returning to Canvas to submit your assignments.
Imagine that thermal fluid plays the same role in plant processes as blood in the human body. It is thus inextricably linked to the optimum functioning of the entire system; if cholesterol deposits congest arteries, then that directly affects the heart, which unfortunately leads to the organism shutting down.
— globalheattransfer.co.uk
Thermal fluids are used to move heat through a system. A system could be composed of solar collectors, storage tanks, heat exchangers, valves, pipes, and more. Through all of these components flow a fluid that carries the heat from one point to another within the system. In a heat exchanger, one fluid flows next to another fluid (e.g., air and water) and heat flows from the hotter fluid to the colder fluid (e.g., from a hot air stream to cold water, effectively cooling the air). In a storage tank, the fluid holds the heat and waits to be pumped to where the heat can be used. In a solar collector, the fluid passes through a tube (or pipe) that is either joined to black, radiation absorbing fins (flat plate collector) or contained within a vacuum glass tube (parabolic troughs) or a combination of both (evacuated tube collector). Selecting a thermal fluid requires knowledge of the physical properties of each fluid under different thermal conditions.
This lesson will take us one week to complete. The list of assignments for this lesson is provided in the table below. More detailed instructions are given on respective pages of this lesson and in Canvas modules.
Tasks | Assignment Details | Access/Directions |
---|---|---|
Readings |
Required:
Supplementary:
This report presents a detailed heat transfer model for tubular absorber system and also includes equations and calculations for heat transfer to heat trasnfer fluids. It is a nice resource to supplement the theory from the D&B book. |
All reading materials for this lesson can be accessed online via links provided. |
Assignment | Comparative assessment of heat transfer fluids | Specific directions are provided on the respective page of this lesson. |
Quiz | Take the Lesson 5 Reading Quiz. | Registered students can access the quiz in the Lesson 5 Module in Canvas. |
Discussion | What makes a thermal fluid the top choice for solar project managers? | Please read directions and post your reflection in Lesson 5 Discussion in Canvas. |
If you have any questions, please post them to our Questions and Answers discussion forum in Canvas. I will check that discussion forum daily to respond. While you are there, feel free to post your own responses if you, too, are able to help out a classmate.
Solar thermal fluids (or heat-transfer fluides - HTF) come in six primary groups:
Each type of heat transfer fluid has advantages and disadvantages with respect to different types of solar thermal energy conversion systems. Oil, water, or molten salts can all be used in Parabolic Trough and Linear Fresnel collector systems, while only molten salt and water (oil is excluded here) in addition to the option of air can be used in a power tower system. Parabolic trough systems are the most widely installed type of system worldwide, at 90% of systems installed by the end of the year 2013 (source: iea.org). Refrigerants and silicones are rarely used in flat plate systems and are not used in concentrating systems for various reasons discussed later.
Oil-based fluids come in three categories; synthetic hydrocarbons, paraffin hydrocarbons, and aromatic refined mineral oils. Water based fluids can be either pure water or a water and glycol mixture with either ethylene or propylene glycol which are types of “antifreeze”. Molten salts are nitrate (ionic) salts that are only available in concentrating systems due to the high temperature requirements of the fluid. Air is standard air, comprised mostly of nitrogen and oxygen, useful for specific applications such as a drying industrial process or low grade heating of building spaces. Refrigerants are commonly used in refrigerators, air conditioners, and heat pumps, but are not commonly used in solar thermal systems today. Silicones can be either synthetic or organic, but are also not commonly used in solar thermal systems today.
There are seven key properties of a thermal fluid for solar application that must be understood before engaging in design work or decision-making regarding thermal fluid performance and/or selection. The properties include:
Learn more about the types and properties of common heat-transfer fluids in the following readings:
Most notably, molten salts bring a high level of corrosion with them. Additionally, molten salts must be kept above their freezing temperature in pipes running to and from collectors or must be drained back from the system components into a holding tank which is configured to deal with the solidification (freezing) of the salt. There has long been a search for molten salts that remain liquid at room temperature, as described briefly in the Advanced Heat Transfer and Thermal Storage Fluids article from the reading. Such salts are anticipated to be low in cost and their high thermal stability (resistance to flammability) makes them very desirable for high temperature applications where alternatives such as oil are highly flammable.
Water-based fluids, such as glycol solutions, degrade over time and must be changed every 3-5 years. If a glycol fluid is subjected to very high temperatures, such as stagnation temperatures when a system is at capacity and the load is not using the heat in the summer, this degradation speeds up, further reducing the life of the fluid. In part, this is one driver for solar air conditioning systems as a way to use excess heat in the summer, increasing the life of the fluid while using free fuel (solar radiation). Pure water is subject to freezing in the winter, but is ideal (very low cost) for locations that do not experience freezing temperatures as well as systems that are equipped (adding additional capital cost) with a drainback tank to hold the water in a thermally controlled space during the times when the collector temperature is below freezing.
Oils are a great fluid for concentrating systems because of their high boiling point (>300 degrees C). With relatively low costs, low freezing points, and high thermal capacity compared to water or air, oils are the best choice for most concentrating systems and are used worldwide.
Some systems use refrigerants, which was discussed briefly above. Chlorofluorocarbon (CFC) refrigerants, such as Freon, have been used historically in some solar thermal systems, but have been phased out due to the negative effect of CFCs on the earth’s ozone layer when CFCs are vented to the environment (either intentionally or accidentally). The benefit of refrigerants is a low boiling point enabling the leveraging of the fluid’s phase change as well as a high heat capacity. CFC refrigerants can be replaced, with some system modifications, by methyl alcohol, ammonia, and more. Research is ongoing in this field.
Silicones is another group of fluids discussed briefly above. Silicones are still rare in solar applications. These fluids require more energy to pump than alternatives due to a high viscosity and they also tend to leak easily through microscopic holes in a solar thermal system. Silicones are interesting and will easily be researched further because of their noncorrosive nature and long lifetime (compared with the 3-5 years of oil).
Flat plate collectors that are intended for domestic hot water and space heating typically use a water-based heat transfer fluid, either pure water if the location is either not subject to freezing temperatures or if the system contains a drain-back tank, or a water/glycol (antifreeze) mixture.
Parabolic trough collectors typically used an oil based heat transfer fluid, though water and molten salt can both be used as well. When water is used, it is used to directly generate steam for use in a turbine or other steam application. Oil can achieve a higher temperature before boiling, thus increasing the efficiency of the collector.
Power tower systems often use molten salt as a fluid of choice. This is because the molten salt does not need to be pumped through a long series of tubes to pass by each collector. The fluid simply resides in the central boiler tank where all of the power tower mirrors are aimed. When the sun sets and the heat is used up, the molten salt freezes (solidifies) in the boiler tank, ready to be melted again when the sun comes up. This greatly reduces the complexities associated with the fluid solidifying inside a series of tubes across a large area with parts that may or may not be easily subjected to solar radiation or other heat sources to melt the salt the following day. Air is typically only used in flat plate collectors, but can also be used in power towers. Air is a good choice for applications such as food/textile drying or space heating.
Please look through the following presentation at the IEA CSP Workshop - it provides additional summary of application of different heat transfer fluids in various solar thermal systems:
Pumping energy is one piece of the puzzle that is important to consider. Pumping power is calculated as the volume of the fluid per unit time (flow capacity) times the density of the fluid times the gravitational constant times the pumping head (vertical distance to be pumped). Pumping energy is simply power multiplied across time. 100kW of power for one hour is 100kWh of energy. Units must be tracked carefully to ensure the correct answer. Friction within the pipe, particularly for pumping over horizontal distances, can be calculated using the Darcy-Weisbach equation (noted in the second video) to relate friction and fluid speed. The head loss due to friction, and as such the required pumping power, is proportional to the square of the fluid speed. As such, this is an important calculation because if your system is designed in a way that requires high pumping speeds, you will have very high pumping energy costs. Additionally, if you pump too slow, you risk damaging your fluid and system components because of high temperatures gained from the solar radiation and not moving that energy through the system, away from the collectors, fast enough. This optimization problem is key to designing a good system.
The video below explains how we can estimate the pumping power required to move the heat-transfer fluid in the system. This kind of calculations becomes handy when you need to determine the cost of using one or another type of fluid for a specific system design. So, watch and see what parameters of the system and fluid need to be taken into account.
PRESENTER: OK, so this is going to be an example of pumping power. And we're going to use water for now. But essentially, the only difference is the density. So for this equation-- and so if you had a fluid that was denser or less dense, for example, like oil, it would be pretty comparable to this calculation. So power is equal to basically the volumetric flow per unit time times the density of the fluid, times earth's acceleration due to gravity-- acceleration due to gravity on earth, which is 9.81 meters per second, you may recall from physics class, potentially even in high school-- times the head of the pipe that you're in. The distance. And that's essentially it. And you have to just be careful of the units. So if, for example, we have fluid that's traveling 1 meter cubed per hour-- that's the volumetric flow rate-- and the density of our fluid is water, so 1,000 kilograms per meter cubed. And we know that g is 9.81 meters per second squared, acceleration. And let's pick a distance of 10 meters. We want to pump our fluid up 10 meters, essentially. What we end up with, 1 meter cubed per hour times the density is 1,000 kilograms per meter cubed-- and those cancel-- times 9.81 meters per second squared and-- oops. Meters did not cancel with anything yet. Sorry about that. And our distance is 10 meters. So you can see here in the numerator we have kilograms, meters squared, per second squared. There's kilograms, meters squared, per second squared. Kilogram, meters squared, per second squared is equal to a joule unit of energy. We also have time still in the denominator over here. So if we have joules per time, that's power, but we need a conversion factor. We need to say that one hour is 3,600 seconds. So then we can cancel hours, and we're left with seconds, and a joule per second. So a joule per second is equal to a watt. So we get it in watts if we do that. So when we multiply all of those pieces together-- handy-dandy calculator here. 1,000 times 9.81 times 10 meters, divided by 3,600. Divided by 3,600. That would be the error I just typed in my calculator. For all I know, you've actually already done this quicker than I have. You end up with 27.25 watts as your pumping power to achieve that flow rate of 1 meter cubed of fluid per hour. That's the continuous pumping power required to do that. So just real quick here. If we wanted to know the energy, say, over a four-hour period, 27 and 1/4 watts for four hours is 109 watt hours, or 0.11 kilowatt hours of energy. So in this case, we weren't pumping too fast. 1 meter cubed per hour is not that high of rate. We weren't pumping it very far either. Only 10 meters up against gravity. So it's pretty low-cost, low-energy result. But as you scale that up over, say, an entire multiple acres of collectors or whatever, you would definitely see high pumping costs. Thanks.
PRESENTER: So in the previous example, calculated the pumping energy and power requirements for pumping water up 10 meters. In most cases in a solar array, there's horizontal piping, not vertical piping. You're piping the fluid through the collector across a big horizontal surface area. So that still requires energy because of the friction in the pipe. So how do you translate that to an equivalent head loss, essentially? And we do this with the Darcy-Weisbach, which says-- let's see here-- Darcy-Weisbach equation, which says that the head loss due to friction is equal to Darcy friction factor, which can be looked up based on your various fluid parameters, such as the Reynolds number, whether your flows turbulent or laminar, things like that, times the length of your pipe. So if you're pumping it 1,000 meters, that would go there and then the internal diameter of your pipe. Oh, back to the Darcy friction factor, that also has to do with the roughness of the pipe. So if you have a very smooth pipe, then you would get a lower friction factor. Times the average velocity of the fluid squared. Divide by 2 times the acceleration on Earth, 9.81 meters per second squared. So you can see already that the head over here is-- h sub f, the thing we're calculating, is affected by the square of velocity, which shows that as speed increases in your fluid, you're going to have a much higher head lost, which from the previous calculation shows you have much higher energy. So if we have a Darcy friction factor of say, 0.2 and that's unitless and we want to say, do a pipe that's 1,000 meters long with an internal diameter of about an inch, 0.03 meters, we're going to do it for two different philosophies here. So let's say that the average velocity for the first round is 3 meters per second. And we know that g is 9.81 meters per second squared. And we plug all these values in-- 0.02 times 1,000 meters, diameter of 0.03 meters. Velocity is 3 meters per second squared. 2 times 9.81 meters per second squared. So you can see some of these units cancel out, meters, meters, seconds squared, seconds squared. We have meters squared, meters in the denominator, so we're going to end up with meters, because there's two meters up top, one below. So in the end, this is equivalent to units of meters. Once we crunch the numbers, times 1,000 divided by 0.03 times 3 squared divided by 2 divided by 9.81, we end up with 306 meters of head loss. So what this says is that one pipe that's 1,000 meters long and one inch in diameter at 3 meters per second has the same essentially energetic requirements as pumping fluid with no friction losses but straight up 306 meters vertically on Earth. For on the moon, it would take less energy, just as a side note, because the moon has 1/6 the gravitational constant. So it does matter these types of things, though I doubt you'll be installing a solar collector on the moon for now. But just a little side note there, all these little details do matter. And so let's see how this changes if instead of 3 meters per second average velocity, we have say, 1/2 a meter per second average velocity. So up here, this 3 would change to be 0.5 and we would run that calculation again. And we would get-- let's see here-- 0.2 times 1,000 times 0.5 squared instead of 3 squared divided by 2 and 9.81. Hold on. I forgot to divide by 0.3-- 0.03, I mean. There we go. That's a better number. There we get about 8.5 meters for the second version of this calculation. And so you can see that by reducing the flow speed to 0.5, we have much less head loss or much less pumping energy required. And so the implications of this are really that you can save a lot of energy by pumping slower. At the same time, in a solar thermal energy collection application, that means your fluid would get hotter a lot faster. And so it could be a good thing under certain circumstances and in others, it could be bad, because your fluid could be overheating if it's going too slow through your collector. So this becomes an optimization problem where you have to balance all these different things happening at once, the fluid flow, as well as the rate of energy being absorbed, as well as what those maximum temperature thresholds are for good operation of your system without damaging the fluid or any other components, as well. That's sort of a fine line that has to be walked to ensure that your system is working correctly. So hopefully, that gives you a little bit of an insight into that on a more technical level. And thanks for listening.
Thermal fluids have several key properties that need to be carefully considered when selecting a fluid for a specific application. These properties include maximum temperature, freezing temperature, density, vapor pressure, specific heat, enthalpy, and viscosity. By performing calculations, one can save a significant amount of money that would otherwise be spent on trial and error experiments and extra equipment, which potentially can be avoided.
This assignment is a series of short analytical problems dealing with thermal fluid assessment. Some problems will require conceptual explanation and some will involved quantitative calculations.
Scenario:
There are two heat transfer fluids - oils that are similar in cost, quality, and expected lifetime. Here are their technical characteristics:
Oil A:
Tmax = 300°C
Tfreeze = -10°C
Cp = 1.7 kJ/kgK (specific heat)
ρ = 800 kg/m3 (density)
Oil B:
Tmax = 395°C
Tfreeze = 13°C
Cp = 2.3 kJ/kgK (specific heat)
ρ = 700 kg/m3 (density)
Range of reasonable Darcy Friction Factors:
0.01 > fd > 0.09 (for problem 3, use fd = 0.04)
Tin = 150°C
Tout = 280 oC
Answer the following questions:
List all your answers in the comparative table Oil A vs Oil B and include it in the end of your report.
The problems can be either typed (size 12 Times New Roman or equivalent font) or hand-written as scanned (like in previous lessons). Please submit your solutions in a single PDF file to Lesson 5 assignment dropbox in Canvas. Remember to show all key equations and analytical steps you take in your solution.
The main function of the thermal fluids is moving heat from the collector to the point of demand in STE systems. Solar thermal heat transfer fluids come in all types and densities (as opposed to shapes and sizes, since that’s not really possible with a fluid). The properties of each fluid make some fluids better for certain applications than others. Sometimes it is a challenge to choose one fluid over another with seemingly equal tradeoffs between the various options. In the end, reliability and historical track records that reduce the risk associated with any single fluid option are typically what steers decisions in the end. As new fluids are researched and developed, we must push forward with systems that use the current state-of-the-art fluids that are proven already.
Please double-check the to-do list on the Lesson 5 Introduction page [9] to make sure you have completed all of the activities listed there before you begin Lesson 6.
Links
[1] http://energy.gov/energysaver/articles/heat-transfer-fluids-solar-water-heating-systems
[2] https://www.e-education.psu.edu/eme811/sites/www.e-education.psu.edu.eme811/files/Lesson8/37083.pdf
[3] http://www.nrel.gov/docs/fy05osti/37083.pdf
[4] https://www.e-education.psu.edu/eme811/sites/www.e-education.psu.edu.eme811/files/Lesson8/FrankLenzenOverviewofParabolictroughslinearFresnekreceivers.pdf
[5] https://www.iea.org/media/workshops/2014/solarelectricity/FrankLenzenOverviewofParabolictroughslinearFresnekreceivers.pdf
[6] http://fac.ksu.edu.sa/sites/default/files/34169.pdf
[7] http://energy.gov/energysaver/heat-transfer-fluids-solar-water-heating-systems
[8] http://www.iea.org/media/workshops/2014/solarelectricity/FrankLenzenOverviewofParabolictroughslinearFresnekreceivers.pdf
[9] https://www.e-education.psu.edu/eme811/node/727