A Solar-powered Fluidyne Test Bed

In our laboratory at the University of Colorado at Colorado Springs, we built and instrumented a solar-powered fluidyne (liquid piston heat engine) test bed. [1]

The test bed was intended to serve two purposes.  First, it was used to demonstrate direct solar-powered operation of a fluidyne with sunlight concentrated directly on the fluidyne cylinder rather than on a remote heat exchanger.  In addition, it was used to characterize some aspects of the operational thermodynamic cycle.  The test bed was only intended as a platform to explore feasibility and thermodynamic characteristics, not as a practical, power producing engine.  The test bed was to be powered by the sun using a 50” by 37” Fresnel  lens. The Fresnel lens supplied ample solar power for the fluidyne.  

Measurements of temperature and pressure inside the working space of the engine showed that a temperature gradient existed across the working space at all times.  

This figure shows about 20 seconds of temperature data from a single test.  During this segment, the temperature fluctuations on the hot side were about 4 °C, and the temperature fluctuations on the cold side were approximately 12 °C.  As expected, the temperature fluctuations from the two thermocouples are out of phase with one another.  

This figure shows a pressure-volume plot of the working space.  The enclosed area represents the work of each cycle.  This engine was operated without any external load for this phase of testing.  Therefore, the indicated cycle work is only overcoming internal friction and other losses.   

The instrumented solar-powered fluidyne test bed demonstrated consistent and repeatable operation.  The Fresnel lens used provided ample energy to power the fluidyne.   Indicated work of the engine was collected with pressure and volume measurements.  Since the engine was unloaded, this work only overcame frictional losses.

[1]  J.W. Mason and J.W. Stevens, 2011, “Design and Construction of a of a Solar-powered Fluidyne Test Bed” Proceedings of the ASME 2011 Mechanical Engineering Conference and Exposition IMECE2011, November 11-17, 2011, Denver, Colorado, USA, paper IMECE2011-62194.

Liquid-piston Stirling Engines

       As I indicated in my last post, one bedeviling factor in many renewable energy applications is the difficulty of making a cost-effective conversion device because it is so hard to make enough money to offset relatively high capital costs. Naturally, improvements in conversion efficiency can help on the money-making side.  Another approach would be to try to pursue technologies where the capital cost (and typically, the efficiency) is low to begin with.  If the primary energy source has a low enough cost (zero, for example) the low efficiency is not as big a concern.  Note, however, that even with free energy and low capital equipment costs, there still remain many other costs such as land, maintenance, transport, etc.  Nevertheless, technologies with low costs for capital equipment are worth exploring [1,2] and may be viable for some niche applications.

       One such technology that may be promising is the liquid piston Stirling engine.  These heat engines can be built very cheaply from a few pieces of pipe and tubing.  While they tend to have very poor efficiency and power density, their very low capital cost could make them economical for some applications using free or low cost sources of heat.

Liquid piston Stirling engines use oscillating columns of liquid, typically but not necessarily, water, in place of the pistons of a traditional Stirling engine.  This figure shows one common configuration (of many possible) with water held in a U-shaped tube and in a connecting “tuning” line.  Oscillations of water in the U-tube shift the working fluid above the water (typically air) back and forth between the hot-side (heat input) and cold-side (heat rejection) of the engine.  Oscillations of the water in the tuning column alternately compress and expand the working fluid.  If the phasing is correct, the expansion occurs when the bulk of the working fluid is on the hot side, and the compression occurs when the bulk of the working fluid is cold, and the cycle produces net work.  The work can be extracted from the oscillations of the liquid in either column, from the pressure fluctuations of the working fluid, or in other ways.  In a properly designed system, the oscillations in both the U-tube and tuning column start moving spontaneously and with the proper phasing when heat input and extraction starts with the working fluid.  An animation at the end of the post illustrates the fluid motions.

         Liquid piston Stirling engines have had limited commercial success to power water pumps and have been proposed for electricity generation in a number of configurations.  West provides a comprehensive overview of the theory and operation of these devices [3,4]. Active research continues on a variety of aspects and applications e.g. [5-7].  Because they have relatively poor power density and efficiency, they would tend to function best for small-scale applications using free or low-cost heat sources such as commercial waste heat or concentrated solar energy.  Since the power output is proportional to the mean pressure of the working fluid as well as the working fluid volume and temperature difference, atmospheric engines using water for pistons have severe limitations on improvements of the power density.  Other configurations are, of course, possible.

On the positive side, liquid piston Stirling engines can be constructed with very low capital cost as indicated previously.  In addition, they are quite robust thermally and mechanically.  They are self-starting with the input of heat, and function well within very loose design and construction tolerances. 

 References

[1] J.W. Mason and J.W. Stevens, 2011, “Design and Construction of a  Solar-powered Fluidyne  Test Bed” Proceedings of the ASME 2011 Mechanical Engineering Conference and Exposition IMECE2011, paper IMECE2011-62194.

[2] J.W. Stevens, 2010, “Low Capital Cost Renewable Energy Conversion With Liquid Piston Stirling Engines,”  Proceedings of ASME 2010 4th International Conference on Energy Sustainability ES2010, paper ES2010-90129.

[3] West, C.D., 1983, Liquid Piston Stirling Engines, Van Nostrand Reinhold, N.Y.

[4] West, C.D., 1987, Stirling Engines and Irrigation Pumping, Oak Ridge National Laboratory Technical Report ORNL/TM-10475.

[5] Orda E. and Mahkamov, K., 2004, “Development of ‘Low-tech’ Solar Thermal Water Pumps for Use in Developing Countries,” J. Sol. Energy Eng. Vol. 126, pp. 768-774.

[6] Slavin, V.S., Bakos, G.C., Finnikov, K.A., 2009, “Conversion of thermal energy into electricity via a water pump operating in Stirling engine cycle”, Applied Energy, Vol. 86, pp. 1162-1169.

[7] Van de Ven, J.D., 2009, “Mobile hydraulic power supply: Liquid piston Stirling engine pump”, Renewable Energy, Vol. 34, pp. 2317-2322.
 

Capital Costs and Renewable Energy

Sometimes people think that because the primary energy is free, use of renewable energy sources should naturally grow to fill most or all of the energy needs of the world.  However, energy from renewable sources currently constitutes a relatively small fraction of total energy use in the United States and worldwide.  As one illustration, consider the electric power generation in the U.S by primary energy source in this figure from several years ago.  Excluding hydroelectric, only about 3% of the U.S. electric power came from renewable energy.  While progress is being made, and other perspectives would give somewhat different results, the main point is that renewable sources currently contribute very little to the overall energy needs of the world.

Why is this so?  Part of the answer lies with the relatively high capital costs associated with renewable primary energy sources.

While renewable energy sources such as wind and solar eliminate fuel costs, they have a low energy density relative to hydrocarbon fuels. Consequently, collection, transformation, and transport costs tend to be much higher per unit of energy than for traditional fuels, and the overall (capital, maintenance, fuel) cost of providing the renewable-sourced energy is often not economical.  While progress in efficiency and infrastructure continues to bring costs down, there is still a long way to go before renewable technologies replace hydrocarbon fuels on a large scale.  

As an illustration, this figure shows consumer prices for small photovoltaic panels as a function of size.  A rough estimate from this admittedly unscientific survey (I just looked up prices for solar panels on the internet) indicates that PV panels can be purchased for around $7000/kW. 

Now, the next figure shows an estimate for payback period as a function of capital cost, interest rate, and selling price of electricity with the assumption of no fuel cost and neglecting maintenance cost.  For a remotely reasonable payback period in the neighborhood of five years, the capital cost would have to be in the neighborhood of $1000-$3000 per kW.  Photovoltaic panels, at least at readily available consumer prices, correspond to an unreasonably long payback (around 18 years) for even the most optimistic assumptions on electricity prices and investment costs.

 So what is the conclusion?  Simple economics dictates that many renewable energy sources currently available are priced out of a competitive (unsubsidized) market by capital cost alone, despite any attractive attributes with regard to fuel costs.  Capital cost for renewable energy conversion technologies will be a primary element in their viability.

Free Surface Profile of a Circular Hydraulic Jump

Impinging jets have excellent heat transfer characteristics and are widely used where a lot of heat has to be moved or removed rapidly.  In the case of a liquid jet directed downward against a horizontal surface, a hydraulic jump forms when the free surface makes a sudden transition from supercritical to subcritical flow.  The jump formed by the radial spread of a circular jet is sometimes called a “circular” hydraulic jump.

This work measured the hydraulic jump surface profile and unsteady fluctuations for several configurations of jet sizes, flow rates and downstream depths.  The downstream depth was controlled by an adjustable height weir shown in the figure as height “w”.

The instantaneous position of the free surface was determined by using a fine wire probe.  A small dc voltage was applied to the water, and the probe completed the circuit between the water and the power supply ground.  The fluctuating surface of the hydraulic jump made it necessary to approximate the jump profile as a statistical representation of a large number of individual measurements.

The figure below shows data for a nozzle diameter of 7.8 mm.  Each symbol represents the dimensionless depth (y/d)  where the probe was in the water 50% of the time.  The vertical error bars on each plotted point represent the 0% and 100% submersion locations.  This  figure provides a representation of the approximate mean location of the free surface, along with the approximate vertical extent of the free surface fluctuations.  As would be expected, the fluctuations are largest near the front edge of the hydraulic jump, and are greater for higher Reynolds number.  The progression from a single jump structure to a double jump structure with increasing downstream depth described by Liu and Lienhard (1993) is clearly visible.

The next figure shows the extent of the vertical fluctuations for several sets of data.  The fluctuations are largest at the front of the hydraulic jump, then drop rapidly and decrease more slowly toward the back of the jump.

 Some measurements were taken in the thin flow layer upstream of the hydraulic jump.  The figure below demonstrates a comparison of some measured layer depths with a correlation of layer depths calculated from velocity measurements in a separate work.  Overall, the comparison shows the same general trends and similar layer depths. 

 

  

References

Stevens, J.W., 1995, “Free Surface Flow Profile and Fluctuations of a Circular Hydraulic Jump Formed by an Impinging Jet,” ASME Journal of Fluids Engineering, Vol. 117, pp. 677-682.

Stevens, J., and Webb, B.W., 1992, “Measurements of the Free Surface Flow Structure Under an Impinging Free Liquid Jet,” ASME Journal of Heat Transfer, Vol. 114, pp. 79-84.

Liu, X., and Lienhard, V.J.H., 1993, “The Hydraulic Jump in Circular Jet Impingement and in Other Thin Liquid Films,” Experiments in Fluids, Vol. 15, pp. 108-116.

Transient Heat Transfer Overview

I first saw a figure like this in Professor Adrian Bejan’s book Heat Transfer  and I really liked the succinct way that it illustrates the ballpark relationship between various transient conduction heat transfer approximations that we commonly use.

In log-log coordinates, we have the Biot number (Bi), which is a dimensionless heat transfer coefficient plotted on the vertical axis, and the Fourier number (Fo), which is dimensionless time, plotted on the horizontal axis.

In a solid which is initially at a uniform temperature and is subjected to a sudden change in the boundary condition, the temperature changes will initially occur (Fo <<1) mostly very close to the surface, and the temperature field can be approximated by the solution for a semi-infinite body.  Professor Bejan calls this the “early” regime, and it is represented by blue shading on the left half of the figure.  If the heat transfer coefficient is high enough (large Bi) or for some reason the surface temperature is specified,  then the solution for a fixed surface temperature (still semi-infinite) is appropriate as shown in a different shade of blue in the upper part of the left side of the figure.

For cases where the conduction within the solid is rapid relative to the heat transfer out, the temperature variation across the body can be neglected.  The entire body can be characterized by a single temperature at any given time and the temperature variation of the body with time can be approximated with an approach called “lumped capacitance”.  The Biot number serves as a measure of when the spatial temperature variation is expected to be small relative to other temperature differences.  Lumped capacitance solutions are appropriate for Bi << 1, or a common rule of thumb that is sometimes cited is Bi < 0.1.  Professor Bejan terms this the “late” regime, and it is represented by the reddish shading in the lower right side of the figure.

For cases where neither the Fo nor the Bi number is very small, the exact solutions must be applied.  Actually, the exact solutions could be applied anywhere, but since they are in the form of infinite series, it is most convenient to use one of the simpler approximations, when appropriate.  These infinite series converge rapidly for large values of time, so a for Fo > 1, it is an excellent approximation to use only the first term of the series.  A common rule of thumb for using only the first term of these series is Fo > 0.2.  A graphical presentation of the temperature distribution based on the first term of these series has become known as the “Heisler Charts”.  The area covered by the first term of the exact solutions, or by the Heisler Charts, is shown in the upper right quadrant of the figure shaded green.

Definitions:

 L_c= a characteristic length of the solid

h = convective heat transfer coefficient at the surface of the solid

k = thermal conductivity of the solid

a = thermal diffusivity of the solid

t = time

Reference
Bejan, Adrian.  Heat Transfer.  New York:  Wiley, 1993.

Considering (or not) Graduate School?

I seem to wind up talking relatively often to senior-level students about the pros and cons of graduate school.   Sometimes these conversations are initiated by students who are considering graduate school, other times I initiate the conversation with very good students who don’t seem to have considered the possibility at all.  Since this topic might be of general interest, I thought that today I would share my thoughts and opinions about graduate school in engineering:

(1)    Pros and Cons

If you are a good academic student (roughly GPA >3.4, although there is a lot of play in this), you should definitely consider graduate school as one option.  It isn’t necessarily right for everybody, but it ought to be something that you look at—if you had 3 job offers, you would weigh the pros and cons of salary, location, potential for growth, quality of company, etc. between the different jobs, then decide on which is best for you at the present time.  Graduate school ought to be one of the options to consider for academically talented students.  If your GPA is less than 3.4, you might still want to consider graduate school if it appeals to you.

a.       Pros:

                 i.      A master’s degree pushes you toward the funner end of the job spectrum  Whether this is even true, and the definition of “funner” are both matters of opinion.  My own opinion is that there is a spectrum of engineering jobs ranging from jobs with very little technical content to jobs with a lot of it.  Usually employers who want/need very little technical content are not interested in paying a slight premium for a master’s degree holder.  Conversely, those who pursued a master’s degree are less likely to want a job with little technical content.  So, it becomes a self-regulating thing.  It is also a general tendency, not an unbreakable rule.  My opinion on “funner” is probably obvious.

                  ii.      More of the same: classes, learning 

Your last four (or five or six) years have probably made you pretty good at succeeding in coursework and learning technical subjects.  Presumably you enjoy that, to some extent, or you wouldn’t be reading this at all.  If you plan your program and choose your advisor well, graduate school should be like the best parts of your undergraduate education in terms of the academic things that you enjoy.

                  iii.      Different things 

Graduate school will broaden and deepen your academic experience considerably.  You might attend a different institution and become acquainted with new professors, new classmates and a somewhat (or extremely) different  academic culture.  You will certainly have a different relationship with your major advisor than you typically had with professors as an undergraduate.  You will learn about and engage in research in a different way and/or to a different extent than you might have as an undergraduate.  In general, your classes might be more rigorous, more focused, and more interesting than your undergraduate classes.

                 iv.      Possibly no more debt 

See #5 Assistantships and Funding.  As a general rule you won’t get rich in graduate school, but depending on your circumstances you might be able to get through it without too much additional debt.

b.      Cons:

                 i.      Delaying your career 

After four years of rigorous study, you are undoubtedly eager to get out and earn a real salary and engage in real engineering.  Graduate school will delay some parts of that for a while.

                 ii.      More of the same 

Graduate school still entails taking classes, doing homework, and reading textbooks.  If you can’t stand any more of that, then graduate school may not be the best choice right now.

                 iii.      Long time to catch up $

A fair comparison between earning potentials of BS and MS degree holders has to measure both from the time of receipt of the BS degree.  Most of the comparisons that I see show that the average cumulative earnings of MS degree holders eventually catch up and pass.  However, this normally takes a long time (10-20 years) and is only an average (may not hold true for individuals).  The potential for making more money is not a logical reason to attend graduate school, in my opinion.

(2)    A Big Reason NOT to Go

Don’t consider graduate school only because you don’t have a job, in general.  This is one of the weakest reasons to attend graduate school, and will probably (not necessarily) wind up as an unpleasant experience for you and for your advisor.

(3)    What You Will Do and How Long It Will Take

Your graduate school classes may not be too much different from the courses that you have taken as a senior.  Many schools allow graduate classes to be used for senior-level technical electives.  In addition, if you do a thesis, (see #7 Thesis vs Non-thesis Options) you will work very closely with your major advisor to plan, carry out, and write up a major research project which will constitute your master’s thesis.  Typically, a master’s degree will take somewhere between 18 and 30 months beyond the BS degree if you are going full-time. 

(4)    Master’s vs. PhD

Some schools will allow top students to enroll in a PhD program with only a BS.  Other schools require a completed master’s degree before acceptance into a PhD program.  Normally PhD requirements are expressed in terms of the number of classes since the BS degree.  If you do get a master’s degree first, definitely choose a thesis option.  Almost everything about completing the thesis will be good preparation for working on a PhD.  If you do have a choice, there are good arguments for doing it either way.  On the one hand, the thesis gives you good experience and preparation for working on your dissertation, and the master’s degree gives you a tangible intermediate achievement if the PhD somehow doesn’t work out.  On the other hand, the time spent on the thesis doesn’t add much to your resume once you have the completed PhD degree.

(5)    Assistantships and Funding

Many times, especially if your GPA is above about 3.80, you will be able to get some kind of assistantship to attend graduate school.  The amount tends to be far less than you would make with a regular job, but for a single person with modest tastes, it is usually enough to make it through school without taking additional debt.  Many schools offer a tuition waiver for students on an assistantship.  A few do not, and in those cases sometimes the assistantship will be configured to cover tuition, as well.  There are a wide variety of assistantships and corresponding expectations of the students.  I’ll describe a few aspects and a couple common configurations, but bear in mind that you may find some hybrids or something completely different.

 Teaching Assistantships usually involve grading for a class, running a laboratory, offering help sessions, or even teaching a full class.  In effect, you are getting a part-time job, somewhat related to your academic area, while you work on your degree. 

Research Assistantships are normally given for work in the research laboratory.  Most often, most of the research in which you are involved relates directly or closely to your thesis, so in effect, you are getting paid to work on your thesis.  Research Assistantships might be funded by the department, which would generally give you a little more freedom in terms of the specifics of your project, or they might be funded by an external sponsor (government grant, industry contract, etc.) in which case you would almost surely be directed quite narrowly in the research subject and direction.

(6)    Finding an Advisor

If you are completing a thesis option, it is my opinion that selecting an advisor is the single most important decision that you will make about graduate school.  It is more important than the school that you attend, and it is more important than the specific topic of your thesis.  Throughout your time in graduate school, you will work closely with your advisor, he or she will direct your research and teach you about doing research, and will declare when you have done enough, and will be extremely influential in the shape of your graduate school experience.  

You should almost always sit down and talk with a professor before signing on as their graduate student.   Talk about assistantships, potential research projects, and their philosophy of graduate school.  Most professors are eager to talk about their research (it is usually more of a problem to get us turned off than to get us started).  If someone is too busy to visit with you, a potential graduate student, that ought to be one alarm for you already.  Pay attention to how enthused they are about their research, whether they are interested in you, how they respond to your questions, and, if you can find out, how they treat their current graduate students.  Finally, it would be a rare occurrence if you could compile enough quantitative  information to be completely sure that this is the right professor, so pay attention to your gut feel and your instincts.  I think that it is a necessary (but not sufficient) condition for you to be very comfortable with your advisor in order to have a great graduate school experience.

(7)    Thesis vs Non-thesis Options

Many programs offer the option of completing a master’s degree with either “course-work only” or with a thesis.  Typically, both options require 30 credit hours (usually 10, 3-credit hour courses).  In the thesis option, six credit hours are usually completed as “thesis credits” leaving you with 8 regular classes.  In “thesis credit” classes, you normally just work on your thesis, with your advisor, as you are doing anyway.  In non-thesis option programs, you usually just take 10 classes and leave with your diploma.

From a strict accounting perspective, it would probably be more efficient to do a non-thesis option.  That is, in terms of total effort, your thesis will probably require a lot more work than completing two regular classes.  However, in return for that effort, you engage in research, focus on a complex problem, review pertinent research that others have done, and present the results of your work orally and in writing in a coherent way.   While the effort is higher than two regular classes, the corresponding benefits are much higher.

So, the bottom line is that if you intend to sometime pursue a Ph.D., completing a thesis is probably a wise choice for the preparation that it will give you.  If you are a full time student, and have the option, completing a thesis is probably a wise choice for the benefits that you’ll receive.  If you are working full time and getting your master’s degree one or two classes at a time, it may not be possible to engage in the concentrated time and effort that a thesis will require and a non-thesis option may make sense for you.

(8)    Other Opinions

These are just a few of my thoughts about graduate school.  You can find a lot of other opinions and advice.  Here are a few sources:

http://www.gradschooltips.com/

http://www.quintcareers.com/considering_graduate_school.html

http://gradschool.about.com/

http://www.graduateschool4you.com/

Best wishes on your decision!

Blog Home

Design of a Low-cost, Active Evaporation System

We wanted a preliminary conceptual design for a low energy, low cost, high volume fresh water evaporation system. 

Figure 1 illustrates the proposed system.  A high pressure pump forces water through an atomizing nozzle.  At the same time, a low-speed fan induces a stream of ambient air into the tank. The air exits the tank through a duct configured to form a vortex inertial separator.  A portion of the atomized water evaporates into the ambient air stream through the tank, while the remainder falls back to the reservoir or is collected in the inertial separator and directed back to the reservoir.

Figure 1.  Diagram of conceptual design

Performance

   The liquid water storage capacity depends only on the design choice of reservoir size.  The evaporation rate is also a design variable dependent on energy usage.  The table below shows two preliminary estimates of evaporation performance and energy usage detailed by fan and pump power for just one possible system configuration.  These are only very rough estimates and have not been experimentally validated.

evaporation capacity	pump power (est.)	fan power (est.)
3 liter/hr	1 W	21 W
1 liter/hr	0.37 W	8.2 W

Advantages

    The proposed system offers a number of significant advantages over conventional evaporation approaches: 

           Design freedom

The combination of a high pressure atomizer with an inertial separator allows great flexibility in system design by permitting trade-offs in individual component specifications.  In general, higher pressure water supply will result in smaller droplet size distributions from atomizing nozzles, however this costs more in pumping power and pump cost.  More expensive nozzles will produce smaller droplets and narrower droplet size distributions. Larger droplets can be produced less expensively in terms of nozzle and pumping costs, but they will require greater residence time for evaporation.  Higher air flow rates and higher air stream turbulence levels increase evaporation rates but cost more in fan power.  Trade-offs between various component costs (nozzle, pump, fan, inertial separator) and between capital and operating (energy) costs can be optimized to give excellent performance at minimum overall cost.  The inertial separator at the exit will insure that liquid water is contained so that wide variations in performance of individual upstream components can be tolerated while still resulting in an acceptable design.

Off–design operation

The same flexibility that allows for tremendous design freedom also permits the design of a very robust overall system.  Many factors including nozzle wear, off-design environmental conditions, water contamination, operator interference, control system failure, etc., can result in system operation at conditions outside of the original design parameters.  Ideally, a system will continue to give acceptable performance away from a single operating specification.  The flexibility inherent in this concept will easily allow for such a robust design. 

Evaporative cooling

In general, the evaporative cooling effect will result in a cool and moist air stream exiting the system.   This may be a primary aim or a desirable side effect.  This system maintains the principal advantage of a conventional evaporative cooling system—low operating cost--while avoiding the disadvantages inherent in maintaining a wetted media for evaporation – mold/fungus growth, higher pressure drop for the air system, and maintenance/replacement of the media.