Refrigerant 718

News flash! A major spill of refrigerant R718 occurred on the Interstate last night!  The refrigerant soaked the highway and surrounding ground.  Luckily, in a few hours it dried up with no effect other than helping nearby plants and small animals.  Because, of course, refrigerant R718 is the designation for pure water.  Today we’ll consider advantages and disadvantages of using water as a refrigerant.

Buoyancy

I used to read to my kids from a popular children's book which featured a curious monkey who got into trouble by grabbing a large bunch of helium balloons from a balloon salesman.  According to the storyline, the monkey was carried out over the city hanging from the balloons.  Now, I'm sure the illustrator for the book was deeply interested in thermodynamics (isn't everyone?) but may have been in too much hurry to calculate the appropriate number of balloons.  Or maybe artistic aesthetics overruled thermodynamic accuracy.  In any case, today we'll go back and look at the situation in more depth.

Water from Air

We’ve talked in other posts about how water will condense out of air when it is cooled past the dewpoint temperature.  So I got to wondering how much it would cost to produce water that way.  In this post we’ll look at a very rough estimate of the minimum cost.

Finite Quench Revisited

We looked earlier at the solution for quenching an object in a finite bath—that is, where the bath is small enough (relative to the object being quenched) that the bath temperature rises while the object’s temperature goes down.  As you’d expect, eventually the object and the bath arrive at the same equilibrium temperature.  Today, we’ll look at getting the quenched object to two specific temperatures at two different times.

Potential for Evaporative Precoolers

In the last post we described a possible configuration for an evaporative precooler to lower total air conditioning operating costs at the expense of some additional capital costs (ducts, heat exchanger, evaporator) and perhaps a little additional maintenance costs on those items.   Of course, in order to make a decision on the payoff time, you have to know your climate conditions, your equipment costs, your operating costs, and the potential savings from the addition of the evaporative cooler.  In this post, we’ll provide a fuller description of the potential benefit you might be able to expect from an evaporative cooler for given inlet conditions.

An Evaporative Pre-cooler

We talked in an earlier post about evaporative cooling and how you can use it to cool off air by simply evaporating water.  Evaporative coolers work best in dry climates, and are much cheaper to operate than vapor compression systems since there is no electricity-hungry compressor.  Unfortunately, they result in a large increase in the relative humidity of the air which may render it unacceptable for some indoor air applications.  In this post we’ll talk about how evaporative coolers are used in combination with other air conditioning equipment to lower the overall cost of providing conditioned air.

Is Incompressible Good Enough?

       In fluids, aerodynamics, and thermodynamics, sometimes it is important to distinguish whether a gas stream needs to be treated as compressible flow or incompressible flow. In this context, these terms have a little different implication than they might have in common use. Gases are pretty much always compressible in the sense that their density changes significantly with changes in pressure. However, in the specialized context of “compressible flow” we are talking about situations where the gas velocity is high enough that the kinetic energy of the flow plays a significant part in determining the properties and changes in properties of the gas. A common rule of thumb is that a flow should be treated as compressible if the velocities involved exceed about 1/3 the speed of sound in the fluid. Of course, that is a general guideline, not a sharp limit. In this post we’ll explore that guideline a little bit.

Compression Ignition Engines

While electric cars are starting to gain a following, the vast majority of the cars on the road today are still powered by internal combustion engines.  The two main types of engines used for automobiles are gasoline engines (spark ignition) and diesel engines (compression ignition).  One of the main differences is that in a spark ignition engine a mixture of air and gasoline fumes is compressed, and lit off by a spark at the appropriate time.  In a compression ignition engine, on the other hand, only air is compressed and a fine mist of diesel fuel is injected into the hot air at the appropriate time.

Refrigerators and Cryotherapy

What does a common household refrigerator have in common with a home medical treatment? One answer might be that you get ice cubes out of your freezer to put on your black eye or sprained ankle. There might be other answers surrounding your treatment when the refrigerator tips over on top of you, but in this post we are going to talk about refrigerator operation and cryotherapy.

Specific Heat Ratio

The specific heat ratio, k, is defined as the ratio of the constant pressure specific heat to the constant volume specific heat.  It depends weakly (over small temperature ranges) on temperature.  In this post, we’ll look at the effect of that temperature dependence.  This figure shows the value of the specific heat ratio of air as a function of temperature.

Compression Heating of a Gas

An ideal gas, like air, or helium, will tend to heat up when it is compressed.  Unless measures are taken to cool the gas during the compression process, this can lead to a pretty large temperature increase for relatively modest pressure increases.  If we assume that no cooling takes place (adiabatic) it is easy to calculate the minimum temperature to which the gas is heated from the compression process alone.

A Thermodynamics Class Project

In my graduate thermodynamics class last year we did a semester-long project developing a computer model of an internal combustion, spark ignition engine.  We started with a simple Otto cycle and through the course of the semester added more realistic effects until by the end of the semester we had a fairly reasonable computer model for an engine.

Keep Turning Up the Hot Water!

In the last two posts, we considered the cooling of the exit temperature of a hot water tank with a given flow rate and heat input rate, and thought about recharge times once the hot water withdrawal was stopped.  However, in real life you’d keep turning up the proportion of hot water as long as you could in order to keep your shower at a constant temperature.  Of course, the cooler the water came out from the hot water tank, the more you’d need to use, making it cool down even faster.

How Long Should I Wait to Shower?

Last post we talked about the temperature of a hot water tank as hot water is withdrawn, and also about the temperature while it is recharging.  With the expressions that we had for those processes, it is possible to calculate the time that it would take for the tank temperature to reach a certain point, and for the tank to recharge from that point. 

Who Used All the Hot Water?

You may have had the wonderful experience of starting a nice, hot shower, only to find that somebody has mostly drained the hot water heater tank.  This leaves you desperately turning the tap to more and more hot water while the shower gets cooler and cooler.  You lather, rinse, (no time for repeat) with feverish speed, but eventually wind up rinsing off in lukewarm (or ice-cold) water.  If so, this post is for you.

Pressure Cookers at Altitude

Water boils at 212 deg F at sea level, of course, but the boiling temperature of pure water is a pretty strong function of the ambient pressure.   This figure shows the boiling temperature of water at different pressures.  Atmospheric pressure at sea level is around 14.7 psi, and that point is marked on the figure with a red circle. The standard operating pressure for pressure cookers is 15 lbs which means 15 psi above atmospheric pressure.  That point is marked with a green triangle in the figure.  You can see that the boiling temperature for water is around 250 deg F inside a pressure cooker operating at sea level.

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.

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.