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Compact phase change based thermal stores: experimental apparatus, methodology, and results.

INTRODUCTION

Electric generation systems are moving from a model of on-demand generation to one of as-available consumption. This is due in part to the increasing use of intermittent renewable energy resources [Hermanns et al., 2009]. Commercial and residential cooling loads account for 11% of all U.S. electric consumption, and controlling the demand for cooling loads will be necessary in an as-available consumption model [USEIA, 2013]. The control of building cooling loads can be achieved through the development and use of localized active thermal storage units.

Studies have shown that active thermal storage systems can be effective at both reducing peak energy and the overall cost of energy use by environmental systems when cost incentives for peak demand and time-of-day rates are in use [Hajiah and Krarti, 2012a,b]. These strategies flatten utility generation curves by reducing peak energy demand, allowing energy to be generated more efficiently by base load power sources without the need to invoke costly and less efficient "peaker" power plants.

Paraffin waxes consisting of one or more compounds of the generic form CH3-(CH2)n-CH3 have been investigated for their use as PCMs (phase change materials) in thermal storage systems [Abhat, 1983]. The use of PCMs allow the storage of thermal energy as latent energy; PCM-based systems store energy at a greater density and over a smaller temperature range than sensible energy thermal stores, such as conventional chilled water tanks. Studies have shown that PCM's with the needed properties of specific melting point and heat of fusion can be crafted using a paraffin with hydrocarbon chains of particular length, or through the use of mixtures of such paraffins [He et al., 1999; He et al., 2004]. These paraffin compounds are stable through their melt-freeze cycles, are chemically inert, non-toxic and possess high heats of fusion. Such paraffins are suitable as phase change materials (PCM) for use in cold storage systems. However, caution is recommended when using binary mixtures or impure paraffins due to the resulting temperature range over which freezing occurs. Failure to account for the affects of temperature range on thermal store performance may result in a system incapable of fully charging or discharging the PCM material during normal operation [He et al., 2004].

A review of phase change systems for latent heat storage found that most PCM materials suitable for cold thermal storage, such as low temperature paraffins, exhibit poor thermal conductivity. This poor thermal conductivity requires encapsulation methods or heat exchanger enhancements that act to improve the overall performance of PCM-based thermal stores [Agyenim et al., 2010]. Smaller encapsulation modules (to reduce the heat transfer path dimension) or encapsulation systems incorporating internal thermal paths must be used to overcome this problem when the limits of PCM performance are reached before satisfactory results are achieved.

This paper presents a study related to the development of a simple, compact thermal storage unit suitable for use in small to medium commercial applications that utilize chilled water cooling coils. This thermal store design is based on the use of a paraffin phase change material (PCM), macro-encapsulated into cylindrical tubes that are densely packed into a containment tank. The encapsulation tube size and wall thickness can be selected as required to overcome the poor conduction characteristic noted with paraffin PCMs and to provide a specific energy delivery rate. The space left between the hexagonal close-packed cylindrical tubes of PCM allow for the flow of a propylene glycol solution HTF (heat transfer fluid), which is circulated through the containment tank to transfer thermal energy with the encapsulated PCM. Thermal energy is stored primarily as latent energy through the heat of fusion associated with phase change of the encapsulated PCM.

An experimental scale model of this thermal store is constructed for testing purposes. Tests are conducted to determine the feasibility of this design, and also to characterize its operation. This information will be used to support future development for a numeric model of the store for optimization and application design guidance.

METHODS

PCM

The PCM material utilized for the experimental model is a technical grade of n-tetradecane. Tetradecane is chosen due to its freezing point being suitable for chilled water storage systems (~ 5.6[degrees]C, ~42[degrees]F), high heat of fusion (~200kJ/kg, ~86 BTU/[lb.sub.m]), and the lack of a subcooling requirement for freezing. A technical grade of 95% tetradecane is selected as a cost concern. Composition of the technical grade product tested is shown in Table 1.

PCM testing

For PCM materials testing, a sample of technical grade n-tetradecane is encapsulated in clear, 1/2 inch (12.7mm) schedule 40 PVC tubing ~ .15m (5.9 inches) in length. The sample is instrumented with a single thermistor temperature sensor placed in the approximate center of the tube. A controlled temperature water bath is outfitted with a separate thermistor temperature sensor, and both sensors are connected to a logging system. For the freeze cycle test the sample is placed into a water bath set to 0[degrees]C (32[degrees]F); for the melting cycle the water bath is set to 10[degrees]C (50[degrees]C).

PCM Encapsulation

The PCM encapsulation system consists of CPVC tubes filled with technical grade n-tetradecane. CPVC was chosen due to its chemical resistance to n-alkane paraffins. Standard V2 inch (12.7mm) CPVC tubing was used for the encapsulation, since this tubing size fits tightly into the 4 inch (0.102m) PVC pipe in a hexagonal close-packed arrangement. Tubes are cut to a length of 1.10m (43.5 inches), which was selected to maintain a manageable size for the test apparatus. Each tube is filled with ~0.1145ml (3.87 fluid ounces) of PCM and fitted with internal endcaps. The completed tubes contain approximately 110mm (4.3 inches) of clear space above the liquid PCM to allow for expansion/contraction of the PCM without large changes in internal pressure.

The use of internal endcaps allows the PCM encapsulation tubes to be fitted tightly together, providing for a high packing density and forming roughly triangular spaces around each CPVC pipe where heat transfer fluid can flow. This arrangement is shown in Figure 1. Note that this arrangement of tubes produces small, 60-degree sections that are identical to one other and have adiabatic interfaces, which will simplify later numeric modeling of the thermal store design.

[FIGURE 1 OMITTED]

PCM tank

The PCM tank is constructed of standard 4 inch (0.102m) schedule 40 PVC pipe with an internal radius of 0.0508m (2.0 inches). The top and bottom of the tank utilize typical schedule 40 PVC fittings to allow for water flow and instrumentation cabling. A PVC pipe flange is installed at a level coincident with the top of the PCM tubes to facilitate installation of the PCM tubes and instrumentation.

The PCM encapsulation tubes are stacked into the pipe in a close-packed hexagonal configuration. As noted above, this leaves roughly triangular shaped flow paths for the HTF. However, at the inside edge of the tank surface the close packing arrangement fails and larger flow paths result. The total area of these flow paths are large with respect to the total flow path between the tubes, and so are partially blocked to provide for uniform heat transfer between the PCM tubes and the HTF. These paths are blocked using V2 inch (12.7mm) closed cell foam backer rod, as shown in Figure 2a.

The main body of the tank, which contains the full height of the encapsulated PCM tubes, is placed into a 12 inch ( 0.305m) cardboard form tube and supported by 4 inches (0.102m) of Styrofoam insulation board at its base. The remaining space in the tube is filled with spray foam insulation. The final result is shown in Figure 2b.

[FIGURE 2 OMITTED]

Before use, the base and upper exposed sections of the tank are wrapped in R-13 h- [ft.sup.2][degrees]F/Btu (2.3 K- [m.sup.2]/W) fiberglass blanket insulation, as are all exposed pipe fittings.

Instrumentation

The center PCM tube of the hexagonal close packed arrangement of the PCM encapsulation tubes is eliminated and replaced by an empty CPVC tube containing thermistors. This CPVC tube is regularly perforated along its length to allow HTF flow through and across the tube. Five sensors are placed in the tube, one each at the top and bottom levels of the PCM in the encapsulation tubes, with the remaining three sensors equally spaced between them. The requirement that instrumentation wires pass out of the tank necessitates that the tank operate at atmospheric pressure, as in an open- bath system. All temperature sensors have a rated accuracy of +/- 0.1[degrees]C (0.18[degrees]F) in the range where they are operated for this experiment.

An additional thermistor temperature sensor is installed in an intermediate tank that provides chilled water to the PCM tank.

Testing Protocol

A chilled water and propylene glycol HTF solution (35% PG by volume) is provided by a small portable chiller unit. The HTF is first pumped by the chiller into an open but insulated intermediate tank, from where it is pumped into the PCM tank using a peristaltic pump. Water is returned from the PCM tank into a tertiary tank, from where it is returned to the chiller. Excess water from the intermediate tank is also returned to the chiller via the tertiary tank.

During discharge mode HTF is pumped into the top of the tank, passing over the encapsulated PCM, and then exits the tank via an overflow system attached to the bottom of the tank. The purpose of this flow regime is to insure that a temperature-gradient, buoyancy-controlled flow is maintained in the tank. This prevents the HTF from taking any specific path, or short-circuit, through the tank. Because of the buoyancy-dominated conditions, the HTF evenly flows through the tank and among the encapsulated PCM. Charging is performed using a HTF flow in the opposite direction to that of discharging to maintain the same temperature gradient direction at all times.

Buoyancy domination of the flow is demonstrated by a ratio of the buoyancy forces to the inertia forces using the Grashof and Reynolds numbers:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (1)

The characteristic length L is taken as the length of the flow area of the PCM encapsulation tubes. By maintaining a sufficiently low flow velocity, buoyancy dominates and the HTF flow is evenly dispersed over the cross sectional area of the tank. Diffusers are used at the tank fittings to prevent jet flow that could bypass the desired buoyancy-dominated flow.

[FIGURE 3 OMITTED]

The system is charged using a HTF input temperature of between 0 and -3[degrees]C (32.0[degrees]F and 26.6[degrees]F). Once fully charged, the tank is preconditioned to an overall temperature state of ~ 1[degrees]C (33.8[degrees]F). The purpose of this preconditioning is to minimize the impact of sensible energy changes on the experimental results, since we are primarily interested in the latent energy storage capabilities of the tank; preconditioning would not be necessary in normal operation.

Discharge cycles are performed using an input temperature of ~ 10.5[degrees]C (50.9[degrees]F) and a flow rate of approximately 100ml/min (0.026 gal/min). The flow rate is based on a scaled version of this system for an application requiring a typical flow rate 4.25L/mm (1.12 gal/min) at a delta T of ~ 5.4[degrees]C (9.7[degrees]F) and a total latent storage of 6.25kWh (21300 BTU). The test system has a latent capacity of ~0.150kWh (512BTU), which scales to a flow rate of ~100ml/min (0.026 gal/min).

RESULTS/DISCUSSION

Freeze/melt point testing of the technical-grade PCM

The results of the PCM freeze and melt temperature testing is shown in Figure 3. The PCM tests show that the technical grade n-tetradecane maintains a sharp freeze point temperature, but that the freeze point is depressed from 5.6[degrees]C (42.1[degrees]F) for pure tetradecane to ~ 4[degrees]C (~39.2[degrees]F) for our technical grade product. Testing also shows that the melt temperature of the PCM is not sharply defined, occurring over several degrees beginning at approximately 2[degrees]C (35.6[degrees]F) and extending to almost 5[degrees]C (41.0[degrees]F).

The performance of n-alkanes as a PCM has been previously investigated [He et al., 2003, Zalba et al., 2003, Choi et al., 1992], and in particular with binary mixtures of tetradecane [He et al., 1999 and Choi et al., 1992]. These studies have shown that impurities in paraffin PCMs can change the phase temperature and latent heat capacity of the material. As a result, the effective melting temperature and phase change enthalpy of this PCM material is best determined by differential scanning calorimeter (DSC) [He et al., 1999], rather than by using published technical data for pure tetradecane. Also, the imprecise melting behavior observed must be considered when developing the numeric model of a thermal store that uses this PCM.

During the PCM freeze-melt tests it was found that a small vertical temperature gradient in or around the PCM tube produced a conical melting pattern in the encapsulated PCM. A gradually tapering cone was formed by the solid PCM. A thermal gradient is maintained in the PCM tank by design, and so this conical melt pattern is likely to be formed in the encapsulated PCM within the PCM tank. This conical melting pattern will drive internal convection currents in the PCM fluid within the tubes, complicating the modeling of the PCM melt cycle.

[FIGURE 4 OMITTED]

PCM tank testing

Both the preconditioning and discharge phase curves are shown in Figure 4. The preconditioning cycle occurs while the PCM is fully frozen, and primarily utilizes conduction for heat transfer internal to the PCM tubes. The temperature curves during this process are non-linear, and become asymptotic as they approach the input temperature of the preconditioning cycle. The discharge cycle initially produces non-linear curves as well, but these become roughly linear after 30 to 60 minutes. The linear portions of these plots terminate at a sharp curve as the temperature of each sensor approaches the input temperature. This is distinctly different than the curves of the preconditioning cycle, and is due to the energy released during the phase change of the PCM.

The discharge cycle of the PCM tank was able to provide a usable temperature delta of >5[degrees]C (9[degrees]F) for approximately two hours of operation when warmed from an initial temperature of ~ 1[degrees]C (33.8[degrees]F) using an input temperature of ~ 10.5[degrees]C (50.9[degrees]F). Over this two hour period the output temperature rose from ~ 1[degrees]C (33.8[degrees]F) to 5.5[degrees]C (41.9[degrees]F). An estimate of the energy output over this two hour period is calculated by numerically integrating over the temperature and flow data, and gives a combined latent and sensible energy recovery of 307kJ (291BTU). This estimate is equal to approximately 60% of the value of the total latent energy capacity for the thermal store (assumes a PCM latent energy of ~ 195kJ/kg or 83.9 BTU/[lb.sub.m]).

A plot of the internal tank temperatures with respect to sensor position and time reveals that a thermal gradient is developed in the tank that travels down the tank as the thermal store is exhausted, as shown in Figure 5. As the gradient intersects the output of the tank, the output temperature rises with the position of the gradient in the tank. Lowering the flow rate, increasing the capacity, or improving thermal transfer rates within the thermal store will increase the slope of this gradient and allow the thermal store to produce a useable output at a more uniform temperature over a longer period.

[FIGURE 5 OMITTED]

SUMMARY

The PCM used in this store needs to be further evaluated to determine its effective melting temperature and heat of fusion. Simple temperature measurements are insufficient, and a DSC should be used to calculate these values for technical grade PCMs.

The test thermal store proved capable of supplying usable energy for cooling purposes, but the overall efficiency (net recovery of thermal energy at a useable temperature over a specific time interval) needs to be improved. This can be accomplished by manipulating overall thermal store capacity, adjusting the encapsulation tube size and quantity, and/or adjusting HTF flow rates. Further experimentation is required to determine the optimum performance parameters for this application.

Future work will use the findings from this study to develop a numeric model of the thermal store so that critical parameters of the design can be identified and optimized. In addition, a larger scale (6.25kWh, 21300BTU) thermal store is under construction to support ongoing experiments and numeric model validation.

ACKNOWLEDGMENTS

We would like to thank the American Society of Heating, Refrigeration and Air conditioning Engineers for their continued support through the ASHRAE Grant-in-Aid program, which has made this research possible.

NOMENCLATURE

Gr = Grashof number

Re = Reynolds number

g = Gravity constant (9.81m/[s.sup.2], 32.2ft/[s.sup.2])

D = Diameter or hydraulic diameter

[beta] = Volumetric thermal expansion coefficient, estimated as 1/(temperature, in Kelvin)

T = Temperature in Celsius, Kelvin, or Fahrenheit

U = Velocity

v = Kinematic viscosity

REFERENCES

Abhat, A. 1982. Low temperature latent heat thermal energy storage: Heat storage materials. Solar Energy, 30 4 (1982):313-332.

Agyenim, Francis, Neil Hewitt, Philip Eames, and Mervyn Smyth. "A Review of Materials, Heat Transfer and Phase Change Problem Formulation for Latent Heat Thermal Energy Storage Systems (LHTESS)." Renewable and Sustainable Energy Reviews 14, no. 2 (February 2010): 615-628.

Hajiah, Ali, and Moncef Krarti. "Optimal Control of Building Storage Systems Using Both Ice Storage and Thermal Mass-- Part I: Simulation Environment." Energy Conversion and Management (April 2012). http://linkinghub.elsevier.com/retrieve/pii/S0196890412000799

Hajiah, Ali, and Moncef Krarti. "Optimal Controls of Building Storage Systems Using Both Ice Storage and Thermal Mass-- Part II: Parametric Analysis." Energy Conversion and Management 64 (December 2012): 509-515.

He, B., Martin, V. and Frederic Settewall. 2003. Phase transition temperature ranges and storage density of paraffin wax phase change materials. Energy, 29 (2004): 1785-1804.

He, B., Gustafsson, E.M., and Fredrick Setterwall. 1999. Tetradecane and Hexane binary mixtures as phase change materials (PCMs) for cool storage in district cooling systems. Energy, 24 (1999): 1015-1028.

Hermanns, Holger, and Holger Wiechmann. 2009. Future Design Challenges for Electric Energy Supply. In Emerging Technologies & Factory Automation, 2009. ETFA 2009. IEEE Conference On, 1-8, 2009. http://ieeexplore.ieee.org/xpls/abs_all.jsp?arnumber=5347150

Kuznik, F., David, D., Johannes, K. and Jean-Jaques Roux. 2011. A review of phase change materials integrated into building walls. Renewable and Sustainable Energy Reviews, 15 (2011): 379-391.

Zalba, B., Marin, J.M., Cabeza, L.F, and Harald Mehling. 2003. Review on thermal energy storage and phase change: materials, heat transfer analysis, and applications. Applied Thermal Engineering, 23 (2003): 251-283.

U.S. EIA Energy Information Administration. "Frequently asked Questions: How much electricity is used for cooling in the United States?". 21 Jan 2012. [Accessed 10 Mar 2013]. http://www.eia.gov/tools/faqs/faq.cfm?id=98&t=3

Stephen Bourne

Student Member ASHRAE

Atila Novoselac, PhD

Member ASHRAE

Stephen Bourne is a Ph.D. graduate student in the Department of Civil, Architectural and Environmental Engineering at the University of Texas at Austin. Atila Novoselac, Ph.D is a professor in the Department of Civil, Architectural and Environmental Engineering at the University of Texas at Austin.
Table 1. Properties of Technical Grade n-Tetradecane

Property Value

Purity, n-tetradecane 95.14%
Branched tetradecanes 2.8%
Less than C14 0.73%
Greater than C14 1.33%
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Author:Bourne, Stephen; Novoselac, Atila
Publication:ASHRAE Transactions
Article Type:Report
Geographic Code:1USA
Date:Jan 1, 2014
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