Chapter 105

AN EXERGY ANALYSIS OF MINE COOLING SYSTEMS

C.T. Twort School of Chemical, Environmental	I.S. Lowndes S.J. Pickering
and Mining Engineering University of Nottingham University Park Nottingham NG7 2RD England	School of Chemical, Environmental and Mining Engineering University of Nottingham University Park Nottingham NG7 2RD England

ABSTRACT

The extraction of minerals and coal at greater depth, employing higher-powered machinery to improve production levels imposes an increased burden on the ability of a ventilation system to maintain an acceptable mine climate. Hence, mechanical mine cooling systems are often adopted, which may be expensive both in terms of their associated capital and operating costs. Consequently, in order to optimise the costs it is essential to provide the mine operator with a method with which to determine the most cost effective and efficient mine cooling system.

The following paper overviews the development of a novel approach to the energy analysis of mine cooling systems using the concepts of thermal exergy analysis. Generic model mine ventilation networks are constructed and the subsurface environments of these mine networks predicted. Models of various cooling system methods are developed and applied to control the underground climate within these mine networks to within pre-set climatic limits. The exergy transfers that are produced by the application of the different cooling methods are compared using performance indices. Models to represent chilled water distribution networks, used to supply the air coolers within the various cooling systems, are designed and balanced. The results of the exergy analyses applied to the operation of the various chilled water pipe networks are discussed and used to assess the exergetic performance of the application of each cooling system to the mine ventilation network.

KEYWORDS

Mine cooling, exergy analysis, cooling strategies, energy efficiency

INTRODUCTION

As mineral extraction becomes deeper with increased levels of mechanisation and higher production rates, the mechanical cooling systems used to maintain an acceptable underground climate become increasingly complex and expensive. To ensure that mining operations may reduce their capital and operational costs it is essential that the industry is able to determine the most cost effective and energy efficient mine cooling system for each operation. This paper summarises the results of an investigation to employ exergy techniques to provide the engineer with an alternative method with which to identify the most practicable and cost effective cooling strategy to control the underground climate.

The paper introduces the general background to the exergy concept and its application. An overview of a study that details the various conceptual mine cooling systems developed and the results of the investigations conducted using thermal exergy analysis is presented. Finally, the analysis techniques developed for the conceptual models are applied to a real case study mine.

The results of the investigations presented build upon the studies previously presented by Twort et. al. (1999).

EXERGY METHOD OF THERMAL ANALYSIS

Exergy analysis brings together the principles of the conservation of mass and energy and the second law of thermodynamics. It is particularly useful in the identification of thermodynamic inefficiencies enabling the location, type and magnitude of energy losses to be determined within a thermal system. The concept of exergy relies heavily on the 2^nd law of thermodynamics and the use of the thermodynamic property of entropy. Exergy establishes the principle that energy has not only quantity, but also quality (measured as the ability to do work). This concept of energy quality may be simply illustrated by considering the effect of burning a finite source of fuel surrounded by excess air, contained within an isolated system, Figure 1.

Figure 1. Illustration of the concept of quality of energy (Moran and Shapiro, 1995)

As the fuel is burnt (figure 1(b)) it warms the air, until finally a warm mixture of combustion products and air remains (figure 1(c)). Although the isolated system still contains the same quantity of energy, the quality of this energy has been degraded. This is because; intrinsically the fuel air combination would have been more useful than the final warm mixture. For example, the fuel could have been used in a device to generate electricity, whereas the potential to recover the heat energy from the warm mixture to produce work is far less. The work potential in the above example was largely destroyed due to the `irreversible' nature of the combustion process. It is this work potential or quality of energy that exergy quantifies. However, unlike energy, exergy can be destroyed and is generally not conserved; though, like many other extensive properties of a thermodynamic system it can be transferred.

In the following sections, the main concepts of exergy theory are introduced.

Exergy Concepts

“Exergy is the maximum useful work that could be produced by the interaction of a system with a specific reference environment” (McGovern, 1990). From this definition, exergy can be termed as the measured departure of a system's state from its reference environment. Hence, exergy it is an attribute of the system and its environment. Therefore, to precisely define the concept of exergy, we need to determine the condition of the environment.

Environment and Dead State: The environment used for an exergy analysis is assumed an infinitely large system that is in a perfect state of thermodynamic equilibrium. This means that there are no gradients or differences involving pressure, temperature, chemical composition, kinetic or potential energy. The environment is also free from any form of irreversibility. Therefore, any system interacting with this environment and which varies in one or more of the above properties has the potential to produce work by coming into equilibrium with its environment. Consequently, it is essential to properly define the reference environment to enable a quantitative exergy analysis of a thermal system to be performed.

To conduct an exergy analysis of a mine cooling system, the reference environment, may for example be that of the atmospheric conditions existing at the mine surface. The reference conditions of temperature T0 and pressure P0 are assumed to be in a perfect state of equilibrium and to represent the typical surface environment.

If a system differs in anyway from its environment (see earlier) then it has the potential to do work. However, as the system changes state and moves towards that of the environment then its capability to produce work diminishes until it reaches a state of equilibrium with its environment. The system is now defined as being in a Dead State and has zero exergy (no potential for work). However, another form of dead state exists when the system is in equilibrium with its environment in terms of its mechanical and thermal states only. This is known as a restricted dead state.

Components of Exergy: The total exergy of a system may be divided into four major components: physical exergy, kinetic exergy, potential exergy and chemical exergy. Since exergy has been defined as the work potential of a system if brought into equilibrium with its environment, then both the kinetic and potential energies of the system can be considered to be fully convertible to work. For example, the kinetic exergy of a circulating chilled water stream is directly related to its velocity, whilst its potential exergy is principally determined by relative elevation, above or below the reference environment.

The physical exergy of a system quantifies the differences in thermodynamic state of the system (T₁,P₁) with respect to its reference environment (T₀,P₀). It is quantified by applying energy and entropy balances between the closed system and its environment. Reversible heat transfer is the only interaction permitted between the environment and the system. Thus, physical exergy is equal to the maximum amount of work available from a system as it is brought from an initial state to the environmental state defined by P₀ and T₀ (Kotas. 1995).

In the application of exergy to the analysis of mine cooling systems, chemical exergy plays no role. This is because normally no chemical reactions, or the mixing or separation of any chemical components take place within a cooling circuit. This fact is accommodated in exergy analysis by assuming a restricted dead state.

The object of employing an exergy analysis, is that all energy transforms within a system may be represented and quantified in the forms of one common parameter, namely exergy. Thus, as a cooling system carries out its various heat transfer and flow functions, the associated energy transfers may be characterised and quantified in terms of gains, losses and destruction exergy. Energy inefficiencies within the system are represented as a destruction of exergy, termed as irreversibility.

The remaining sections of the paper are now devoted to the presentation and discussion of the results obtained from an application of thermal exergy analysis to mine cooling systems.

APPLICATION OF EXERGY ANALYSIS TO MINE COOLING SYSTEMS

Model Mine A, Figure 2, was developed, climate control zones (CCZ) identified and their thermal environment regulated using a spot cooling strategy. The maximum climatic limit permitted within the defined CCZs was 28°C effective temperature (ET). Full details behind the construction, development and cooling of model Mine A may be found in Twort et. al. (1999).

Figure 2. Model Mine A, a coal mine consisting of three working districts and associated developments

In order to absorb heat being removed by the spot air cooling strategy, various cooling systems were investigated. The concept of an ideal thermodynamic cooling system was initially developed and defined as a coolant stream that follows the path of a mines composite cooling curve exactly, to match both its temperatures and thermal capacity. For the detail behind the development and definition of composite cooling curves, see Twort et. al. (1999).

The ideal thermodynamic cooling system defines a datum against which the relative performance of other cooling systems can be compared. The ideal thermodynamic cooling system established the maximum exergy output and minimum exergy input a cooling stream can posses when operated in an ideal manner. Two further `ideal' systems were subsequently developed employing the concept of a minimum approach temperature (MAT);

Ideal series cooling system: is represented by a single coolant stream, which flows at a constant mass flow rate and is capable of following a mine composite cooling curve, without contravening the imposed MAT.

Ideal parallel cooling system: this system employs numerous parallel coolant streams, which can possess variable thermal capacities in order to maintain a temperature difference, equal to the MAT, between themselves and the defined mine composite cooling curve.

These ideal model systems allow a benchmark against which the performance of practical systems may be compared, with the introduction of a temperature differential, which allows heat transfer to occur without the conceptual use of infinitely large heat exchangers. Thus, the evaluated thermal irreversibility (exergy destroyed) resulting from heat transfer, evaluated between the ideal thermodynamic system and the ideal series and parallel systems, represents the unavoidable irreversibility of a cooling system when it operates in an ideal manner under real thermodynamic design conditions.

Using the concept of thermal irreversibility, where the ideal thermodynamic system represents zero irreversibility, it was demonstrated that each district of Mine A, using the ideal parallel system, had a lower thermal irreversibility than using the ideal series system, Figure 3. Therefore, a parallel system always has the potential to thermally out-perform a series system.

Figure 3. The thermal irreversibilities of various
district cooling systems

The exergy transfers were then determined for a number of practical cooling systems that employ series and parallel cooling streams. Using the ideal thermodynamic system as the datum, the thermal irreversibilities of the various practical cooling systems could be evaluated. It may be clearly seen, from Figure 3, that under the operational conditions investigated, the parallel cooling system (FDM) out-performed the series cooling systems (FDM, FMD) by producing a lower thermal irreversibility.

However, it should be noted, that the total thermal irreversibility of a system was determined using its associated exergy transfers, evaluated at an environmental temperature, T_o, of 20°C. Consequently, if its exergy transfers were to be re-evaluated using a different value of T₀, then their values would change, but the total thermal irreversibility would remain constant. This is because the irreversibility represents the differential between the different exergy transfers.

FURTHER APPLICATIONS OF EXERGY ANALYSIS

The concepts and conclusions drawn from the initial model studies presented above were subsequently further developed. In particular, more realistic spot air cooling strategies were applied and parallel cooling systems developed under various design conditions. An analysis of the performance of these systems using exergy analysis and the development of possible performance criterion are presented. To conduct further analyses, model Mine B (Figure 4), based on Mine A, was developed.

Figure 4. Ventilation schematic for conceptual model Mine B

The mine climate was predicted and then cooling applied using four different cooling methods constrained, in turn under two climatic indices; 28°C ET and 28°C Dry-Bulb (DB). The different cooling methods used were as follows:

Theoretical minimum cooling; (Minimum)

The application of this technique serves to determine the minimum cooling duty required to maintain the ventilation air within the CCZs at below the required climatic limit.

Standard spot cooling; (Standard)

Similar to bulk cooling, this technique lowers the temperature of the total air stream entering the CCZ to a level, such that its exit temperature does not exceed the set climatic limit.

Restricted Temperature spot cooling; (RT)

The RT cooling method employs the same techniques as used in the Standard cooling method, but restricts the cooler exit air temperature to a minimum of 20°C dry-bulb. This minimum exit air temperature is chosen as at lower exit temperatures miners working in close proximity to the cooler may suffer from thermal stress.

Restricted Temperature & Volume spot cooling; (RTV)

The RTV cooling method represents the typical air cooling technique employed within the German coal mining industry. As in the case of the RT cooling method, coolers have a restricted exit temperature of 20°C dry-bulb. However, rather than bulk cooling the airflow entering a working the quantity of ventilating air cooled by each cooler is also restricted.

With the exception of the Minimum cooling method, the Standard, RT and RTV spot cooling methods became more representative of the air cooling method used in German mines. For all the cooling methods considered the total cooling duty required under the 28DB limit was at least a third greater than that required for the 28ET limit.

It was concluded that to cool Mine B with the RTV cooling method, required a large number of in-line face coolers. Hence, a variant of the RTV cooling method, RTVa, was modelled to reduce the number of in-line face coolers but which resulted in a deterioration in the thermal climate experienced in the last section of the longwall face. Subsequent cooling was then required in the tailgate of the face to control the climate within the CCZ.

An exergy analysis was then applied to each cooling method, and the exergy transfers determined. The index of Potential Exergy Loss (PEL) (1) performance index was developed as a measure of the performance of an applied cooling method compared to that of a specified ideal cooling method. The Theoretical Minimum Cooling was chosen to represent the ideal cooling method.

(1)

In the application of a thermal exergy analysis and the formulation of the PEL index, the true energy transfers in terms of both their quantity and quality were able to be assessed for each of the cooling methods. Since the minimum cooling method represents the ideal case, then it is considered to have zero PEL. Thus, the smaller the determined PEL for the given cooling method the better was judged its thermal performance. Under the 28DB limit the PEL decreased from the application of the standard cooling method through to the RTVa method (Table 1), as a consequence an of increase in the average heat transfer temperature. This was as expected, since the cooling methods duties decreased in the same direction.

However, although the cooling loads applied were observed to decrease in the same direction under the 28ET limit as in the 28DB limit, their associated PELs did not (Table 2). This was the result of the combined parameters used to evaluate the ET. When the performance of the various cooling methods were compared by evaluating their PEL, it was seen that under the 28ET limit that the RT slightly out performed the RTVa. Under the 28DB the RTVa had the lowest PEL followed by the RTV.

Table 1. The exergy transfers produced by the application of each of the cooling methods under the 28DB climate limit

Table 2. Cooling methods exergy transfers under the 28ET climate limit

A performance index named the exergetic cooling ratio (2) was then introduced.

(2)

The higher this ratio, the more exergy is transferred out of the airstream per kilowatt of cooling. Again, the reference value was set as the exergetic-cooling ratio of the minimum cooling method. Under both climate limits, the RTVa cooling method performed the best.

Parallel cooling streams were then developed to cool the RTVa cooling method, under both 28ET and 28DB climate limits. Two MATs were employed, 13°C and 10°C, for which the ideal parallel cooling systems were evaluated using both fixed and variable chilled water flow systems. Two different chilled water supply temperatures were also used 3°C and 7°C. These variables showed how a system would always have an unavoidable thermal irreversibility whose value depended on the design criterion. Practical parallel cooling systems were then developed and investigated, using a range of operational parameters. The total thermal irreversibility of each system was then calculated. Using the previously determined unavoidable irreversibilities, the avoidable irreversibility of each system was determined, Figure 5.

Figure 5. Avoidable thermal irreversibility for particular parallel cooling systems

The avoidable thermal irreversibility represented the thermal inefficiency of a practical parallel system compared to the ideal parallel system.

The analysis showed how the various cooling systems operated when compared to their ideal parallel. As expected the systems with variable flow rates had lower avoidable irreversibilities than those operating with fixed flow rates. This occurs because the variable flow systems had a smaller average temperature difference over which heat transfer took place. It was also noted that although a system may possess a low total irreversibility, it may perform inefficiently as compared to its ideal.

Cooling system chilled water flow rates were then determined and an investigation conducted to ascertain if the cooling water could be reused. It was found that for fixed flow systems exit water from the in-line face coolers could be reused. It was observed that the thermal irreversibility for systems employing water reuse did decrease. However, the changes produced by such a reduction in the water flow rate may have a more significant effect on the exergy transfers and irreversibility of the pipe distribution network of a cooling system.

EXERGY ANALYSIS OF CHILLED WATER DISTRIBUTION SYSTEMS

This section details the construction and application of the modelled chilled water distribution networks developed to cool the RTVa cooling method. They were designed using the parallel cooling system concept developed earlier. The exergy transfers and irreversibilities of the chilled water streams were evaluated as they flowed through the air coolers and pipe circuit.

Each network was designed, constructed and balanced using a commercial hydraulic network solver. The operational parameters of the various flow components used to model the network were chosen to represent those most commonly employed in German mine cooling systems. Under each climate limit, 28DB and 28ET, three different cooling networks were developed. A sample network is illustrated in Figure 6.

Figure 6. An example layout of a model chilled water system

These networks allowed an investigation to be conducted to compare the different exergy effects produced between the reuse and non-reuse chilled water systems employing the 2m/s optimum water velocity parameter suggested by Van Vurren (1975).

As expected networks with the largest cooling water flow rates required the highest pump power to overcome the systems head losses. However, it was shown that on the implementation of water reuse without a re-sizing the pipe network, that the pumping power requirement of a network could be reduced by over a third. If the pipe network was then re-sized in an attempt to obtain the 2m/s optimum water velocity, the pumping power still remained below that consumed by the original network. This trend was observed under the application of both climate limits, although not as prominently under the 28DB limit.

For all of the networks studied, which employed water reuse, it was noted that although a resize of a pipe network to obtain the 2m/s optimum water velocity increased the pumping power, it would decrease the capital cost of the system by the use of smaller pipe diameters and less insulation.

In the initial analysis of the chilled water networks, only the friction head loss was evaluated. Therefore, only the changes in the exergy input at the pump, due to friction irreversibility were taken into account. Consequently, no allowance was made for the thermal exergy transfers that accompanied the water as it flowed through the network, or the thermal changes that may further affected the frictional head loss. Hence, a pipe exergy model was developed to include all of the thermal changes occurring within the water streams and to evaluate their associated exergy effect.

The pipe exergy model was used to evaluate the exergy transfers and irreversibility as the chilled water flowed around the RTVa cooling networks constrained under the 28ET limit. The results of the network exergy transfer were represented in both a traditional tabular form and pictorially using a Grassmann diagram, an example of which is shown in Figure 7.

Figure 7. Simplified Grassmann diagram representing exergy changes for water network

It was concluded that the total system head loss calculated for each network, using the pipe exergy model, was slightly lower than those evaluated using the hydraulic network solver. This was the result of the use of a lower water viscosity on the return side due to the higher water temperatures.

An examination of the irreversibilities associated with the various components of a network concluded that the pipe friction losses dominate, contributing over 65% of the total irreversibility. Both coolers and mixing points play a relatively small part, with the flow control valves accounting for approximately from 12 to 25% of the irreversibility of a network depending on the design.

Thus, the heat transfer experienced across coolers and pipe irreversibility account for over 80% of the exergy changes observed across a cooling network under the conditions studied.

Exergetic efficiencies of 42.3%, 51.5% and 48.8% were obtained for the networks analysed. However, it was noted that the performance criterion was not valid under all operating conditions.

(3)

where;
exergy flow out of the system

exergy flow into the system

Hence, it was concluded that the performance of the various networks could only be effectively assessed by comparing the thermal exergy transfers and the irreversibilities produced by each system subject to similar operating conditions.

CASE STUDY: MINE COOLING EXERGY ANALYSIS OF A UK COLLIERY

In the following sections a model of mechanical mine cooling is applied to the climatic simulation of an operational UK mine, Maltby Colliery, Figure 8. The climatic model of the Maltby mine was developed using a climate prediction program and measured climatic and ventilation survey data, which allowed the climate of the model mine network to represent the conditions currently being observed underground.

The 28ET climate limit was imposed on the colliery network. It was observed that this climate limit was exceeded within the CCZs of the longwall face and tailgate of T05 s district and the in the T16 development heading.

The minimum cooling method and the RTV cooling method, previously identified as the best and most practical cooling method, were then applied to regulate the air conditions within the CCZs, requiring 757 kW and 1016 kW of cooling respectively.

Exergetic composite cooling curves were constructed to represent the application of each cooling method and their respective exergy transfers determined. It was found that under model conditions, the RTV cooling method had a greater exergy output than the minimum cooling method, whilst also having no exergy input. This resulted in the potential exergy loss (PEL) index of the RTV method having a negative value. However, with the Minimum cooling method chosen to represent the ideal case, the lowest valid value for a PEL for alternative methods is zero. Hence, the PEL index was shown not to hold for all conditions. The exergetic cooling ratio parameter, remain valid. It was observed that the Ideal cooling method (minimum cooling) transferred 16% less exergy out of the air per kilowatt of cooling than the RTV cooling method, as compared to the ideal.

Figure 8. Simplified Maltby colliery ventilation network

Parallel cooling streams were then used to absorb the heat being removed from the air using RTV cooling method. Initially, the ideal parallel cooling system was evaluated, to give a practical ideal datum with which to compare the performance of other parallel cooling systems. This analysis showed that, under the design parameters employed, the intrinsic irreversibility value for any parallel cooling system in the study would be 37.6 kW.

Four practical parallel cooling systems were then developed, their total irreversibility determined and avoidable irreversibilities compared. As expected, the systems with variable flow rates had significantly lower irreversibilities, Figure 9.

For each parallel system, a chilled water reticulation network was constructed and balanced using hydraulic network solver. As in the previous analyses, this showed the reduction in pumping power if a network was converted for water reuse.

The analysis also showed just how small the pumping power of a system could be should low water supply temperatures be used in conjunction with variable cooler water flow rates.

Each network was then subjected to an exergy analysis using the pipe exergy model. An examination of the results of this analysis highlighted the dramatic effect that the different water flow rates and supply temperatures have on the exergy transfers and irreversibilities as the water flows through the network. As observed in the network exergy analyses performed earlier, the thermal exergy transfer was dominated by the coolers, over 90%, whereas the irreversibility is dominated by pipe friction loss, 70% or more.

Figure 9. Maltby Colliery's modelled parallel cooling systems irreversibilities

DISCUSSION

The objectives of the study were to develop and apply thermal exergy analysis to investigate mining cooling methods, cooling transfer systems and coolant transport networks. This was conducted to gain a greater understanding and knowledge of the exergy transfers and destruction that may occur within subsurface mining cooling systems. The results produced by the subsequent investigations identified that the RTVa cooling method and the parallel cooling water distribution system produce the best cooling methods. The study developed the concepts of an ideal cooling method and cooling systems and established a series of design guidelines. The development of universal performance indices and criteria was more challenging. As most heat transfers within mine cooling systems occur across the environmental temperature this created the problem of identifying as to what is a `useful' change in exergy. This was clearly demonstrated in the development and application of the potential exergy loss (PEL) index, which was ultimately found not to be applicable for all the conditions under which cooling is applied. However, the proposed exergetic cooling ratio did provided a good means of comparison of the performance of cooling systems under the conditions studied. The use of avoidable thermal irreversibility to compare various flow configurations and design parameters also enabled exergy analysis to demonstrate that parallel cooling systems will out-perform comparable series systems.

The development of the pipe exergy model demonstrated that the reuse of cooling water in fixed flow system could greatly reduce the frictional irreversibility experienced. The results of these investigations concluded that under the conditions investigated there was very little unwanted exergy transfer through insulated pipes. The main exergy transfer observed was across the coolers, with pipe network dominating the frictional irreversibility experienced across the network.

To assess the performance of mine cooling systems, consideration must be given to all the areas identified in this study; the performance of a cooling method (exergetic-cooling ratio), the thermal irreversibility generated across the coolers, and the results of the exergy analysis on the cooling network. However, ultimately the application of an exergy analysis in the

investigation has revealed the relative magnitudes and nature of the exergy transfers and irreversibilities within a subsurface mine cooling systems. Thus, in general terms, the performance of a system may be judged against these evaluated exergy parameters. However, in order that the results of the exergy analyses may be of practical use, it is necessary to change from an analysis to an optimal synthesis to determine the optimum cooling system configuration. Consequently, this would require the application of thermoeconomic optimisation procedures.

ACKNOWLEDGMENTS

The authors wish to acknowledge the financial assistance of the ECSC. The views expressed in this paper are those of the authors and not necessarily those of the sponsors.

REFERENCES

Moran, M., and Shapiro, H.N., 1995, Fundamentals of Engineering Thermodynamics. John Wiley & Sons, United States

McGovern, J.A., 1990, “Exergy analysis - a different perspective on energy Part 1: the concept of exergy,” Journal of Power and Energy, 204, pp. 253-262.

Kotas, T.J., 1995, The Exergy Method of Thermal Plant Analysis. Krieger, London

Twort, C.T., 1999, “Novel Approach to the Energy Analysis of Mine Cooling Strategies”, Proceedings, 8^th U.S Mine Vent. Symp., J.C. Tien, ed., University of Rolla, Missouri, pp. 349-356

Van Vuuren, S.P., (1975), “The Optimisation of Pipe Sizes in a Refrigeration System,” Journal of the Mine Ventilation Society of South Africa, Vol. 28, pp 86-90

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AN EXERGY ANALYSIS OF MINE COOLING SYSTEMS

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