Ben Costelloe

Indirect evaporative cooling potential in air-water systems in temperate climates

Recent developments have prompted a review of evaporative cooling technology as an effective means of cooling modern deep plan buildings. Prominent among these developments is the success of high temperature sensible cooling systems, particularly, chilled ceilings, which require a supply of cooling water at 14–18 8C. Crucial to the success of evaporative cooling technology, as a significant means of cooling in modern applications, is the ability to generate cooling water, in an indirect circuit, at a temperature which closely approaches the ambient adiabatic saturation temperature (AST) or wet bulb temperature (WBT). Recent experimental research has demonstrated that it is technically viable to generate such cooling water at a temperature of 3 K above the ambient AST. While the frequency of ambient AST occurrence can be obtained from meteorological sources, there is little in-depth analysis of the potential for this form of cooling water generation, based on the approach temperatures which have now been shown to be viable. The decision to use an evaporative cooling system depends largely on an assessment, in-depth, of net energy saved against capital expended. Such an assessment requires detailed data on the availability of cooling water, generated by evaporation, for each location. This paper quantifies evaporative cooling availability in-depth for a northern and southern European city, Dublin and Milan and suggests a method of analysing such data for any world wide location, for which suitable meteorological records are available. The paper, incorporates recent experimental research findings and bases the availability analysis on meteorological test reference weather year data. The results of this research confirm a major potential for the generation of cooling water by evaporative means, which can be used to provide effective cooling of deep plan buildings by means of contemporary water based sensible cooling systems, such as fan coil systems, radiant chilled ceiling panels and ceiling cooling convectors (chilled beams). While the technique offers most potential in locations with a northern European temperate climate, it has significant potential to contribute to cooling in some southern European cities, during the nonsummer months and also at other times, particularly where load shaving techniques are incorporated. # 2002 Elsevier Science B.V. All rights reserved.

Experimental energy performance of open cooling towers used under low and variable approach conditions for indirect evaporative cooling in buildings

The success of chilled ceilings and displacement ventilation systems as a means of sensible cooling in buildings has prompted a review of evaporative cooling technology as an effective means of generating the required cooling water. When such cooling water is generated at low approach conditions (2–5 K), at the higher temperatures required in these systems (14–18°C), very high levels of availability result. In many north western European locations the levels of availability are such that the prospect of supplanting rather than simply supplementing the refrigeration system, for sensible cooling purposes, arises. The viability of the technique, however, largely depends on achieving low approach conditions, at acceptable levels of energy performance. Hence the need to investigate the energy performance of the process. This paper presents the results of recent experimental research into: i) the achievement of low approach conditions in an evaporative cooling test rig; and ii) the energy performance of this test rig when generating cooling water, indirectly, at the temperatures required for chilled ceilings. Energy performance is presented for a range of specifi c conditions and typical annual effi ciencies of cooling water generation are determined. Results are compared with typical energy effi ciencies of conventional, vapour compression based, refrigeration systems. A signifi cant potential for improved annual energy performance, is shown.

The Design and Performance of an Evaporative Cooling Test Rig for a Maritime Climate

Recent developments have prompted a review of the use of cooling tower based evaporative cooling technology as an effective means of cooling modern buildings. Prominent among these developments is the success of high temperature cooling systems such as radiant ceiling panels and chilled beams. At present, however, there is little published literature which gives a quantitative, in depth analysis of the performance or energy efficiency of cooling towers, used in maritime climates, in conjunction with heat exchangers and run at low approach and low wet bulb temperatures throughout the free cooling season. This lack of knowledge has meant that many current opportunities to benefit from the technology are not availed of by building design teams. To address this issue an automated laboratory test rig has been specifically developed with the aim of optimising the performance and demonstrating the potential of this form of cooling in maritime conditions. This paper, which reports on work in progress, describes the design and development of the rig and presents and analyses the preliminary test results. 1.0 Introduction Traditionally, the cooling requirements of buildings have been met by convection cooling employing chilled water systems. In commercial buildings such cooling has largely been generated by vapour compression refrigeration systems producing chilled water at 5 to 8 C. The dominance of convection cooling systems has been reduced in recent years by the success of radiant cooling in the form of chilled ceiling panels and beams (1). Chilled ceiling panels and beams typically require a supply of cooled water at 15C with a return temperature of 18C. Elevated chilled water temperatures raise the possibility of generating the required cooling in cooling towers. Hence the view has developed that cooling tower based evaporative cooling systems ought now to be the subject of a major review, as a practical and low energy means of cooling modern buildings (2,3). Three parallel developments have reinforced this view. As a The Design and Performance of an Evaporative Cooling Test Rig For a Maritime Climate. Dublin 2000 “20 20 Vision” Joint CIBSE ASHRAE conference, Royal College of Surgeons, Dublin 21-23 Sept., 2000. 2 result of better fabric insulation, lower glazing levels and more successful use of solar shading the cooling load in the modern deep plan office building is now dominated by the internally generated sensible load. The cooling season in such buildings is not confined to the Summer months, but extends into periods of the year with good evaporative cooling availability due to the lower ambient wet bulb temperatures experienced. There has also been, in recent years, a trend towards increasing sophistication in the design of towers and fluid coolers. The recent introduction of the hybrid closed cooling tower, which integrates dry, adiabatic and evaporative cooling in one unit, is a case in point. Inverter driven variable speed fans are also increasingly used in cooling towers as a means of optimising energy consumption in chilled water plant (4). Since Legionnaires’ disease was first recognised in 1976, there has been widespread concern about the health risks associated with cooling towers. It is now accepted, however, that the prevention of Legionnaires’ disease in cooling towers is a matter of quality assured maintenance and operating procedures (5). An important feature of cooling towers which are used exclusively in free cooling applications is that the extent of the risk of growth of legionella in such towers is far below that of towers used in cooling refrigeration condensers. This is due to the fact that the water temperature in such towers is generally below 20C. In the course of the current research programme, the following aspects have been identified as requiring detailed research and analysis: 1. The optimisation of tower design for free cooling applications in maritime climates. 2. The optimisation of tower based free cooling systems for energy consumption. 3. The energy efficient control of tower output, particularly during Winter operation. 4. The evaluation of cooling potential using meteorological data for various locations. 5. Environmental and safety issues, such as water consumption and water quality. To address these issues an automated laboratory test rig has been developed as one element of a research programme devoted to this form of cooling. The objectives of the research are to demonstrate the potential and optimise the performance of this form of cooling in modern buildings, located in maritime climates. The current paper describes the design of the test rig and analyses the initial test results. The Design and Performance of an Evaporative Cooling Test Rig For a Maritime Climate. Dublin 2000 “20 20 Vision” Joint CIBSE ASHRAE conference, Royal College of Surgeons, Dublin 21-23 Sept., 2000. 3 2.0 Background The standard approach to indirect water side free cooling is to treat the system in the context of a large water cooled, normally centrifugal, chilling plant, which is designed to serve conventional air conditioning loads at standard temperatures, typically 7°C. The system is usually designed as a changeover system, which, by routing the cooling tower water and the cooling load water through a plate heat exchanger, bypasses the condenser and evaporator circuits, and thereby provides cooling without operating the refrigeration compressor. Opportunities for free cooling are typically seen to arise, when the sensible cooling load reduction and the diminished requirement for dehumidification, occurs, in the off-peak months in conjunction with lower ambient wet bulb temperatures. The extent of the annual energy savings which can be achieved vary with each project, but are typically of the order of 30% with a payback period of under 3years. This is the general approach of most recently published work in this field including De Saulles (3) and Murphy (6). In the literature two main strategies are advocated for maximising cooling availability – raising the chilled water temperature as the seasonal cooling load falls and the use of additional cooling tower capacity as a means of reducing the cooling tower approach temperature (the temperature difference between the cool water exiting from the tower and the ambient air wet bulb temperature). Additional tower capacity can be provided either by multiple towers or alternatively by the selection of the tower on the basis of the free cooling duty with an approach of, typically, 3°C, as opposed to selection on the basis of the Summer duty with an approach of 6 to 10°C. Generally, for indirect free cooling systems, a combined approach of 5°C is advised, composed of a 3°C approach at the tower with a 2°C approach across the heat exchanger. De Saulles (3), presents a BSRIA modelling analysis by extrapolating from the performance of a typical cooling tower which has been sized for normal Summer cooling duty. This analysis states that a tower selected on the basis of a 2°C approach temperature will have an “oversizing margin” of 400 to 500% above a tower selected on the basis of Summer conditions. Murphy (6), also advocates selecting on the basis of the free cooling duty with an approach of 3°C and presents data for one tower producing water at 13°C at an ambient wet bulb of 9.5°C. However the general lack any published test data and in-depth analysis, or optimisation, for cooling towers at low approach, low range, and low wet bulb temperatures is not addressed other than to state that data can be obtained from manufacturers on a “project specific basis”. The Design and Performance of an Evaporative Cooling Test Rig For a Maritime Climate. Dublin 2000 “20 20 Vision” Joint CIBSE ASHRAE conference, Royal College of Surgeons, Dublin 21-23 Sept., 2000. 4 At present there is little detailed reference, or analysis, in the literature, on the application of free cooling technology to water based high temperature sensible cooling systems in general and to chilled ceiling panels and chilled beams in particular. Such systems offer major free cooling potential due to the high cooling water temperature used and the fact that they are designed as sensible only cooling systems. With such systems, as Table 1 shows, the free cooling availability of chilled water at 15°C can be as high as 80%. Hence, if a combined approach temperature of 3°C is achieved the system will operate in the free cooling mode for the major portion of the year, where ambient conditions are suitable. This requires a change in the standard design approach. With high temperature cooling systems it is more appropriate to design the heat rejection system to optimise free cooling than to dissipate condenser heat. An essential question arises in the case of the recently developed induction chilled beam, which can operate at chilled water temperatures as high as 18°C, as to whether the cooling tower system could be designed to provide year round cooling. In the case of the chilled ceiling panel a similar question arises on the impact on the room conditions of allowing the system to operate on a free cooling basis throughout the year. This would require a very low combined approach temperature of 2 to 3°C but would remove the changeover requirement, simplify system operation and allow the tower to be optimised for a single function. If a 3°C combined approach is achieved, the analysis presented in Table 1, suggests that conventional chilled water temperatures of 7C can be produced by cooling towers, on average, for 22% of the year in Dublin, Ireland. On the same basis, temperatures of 15C can be produced for 81% and temperatures of 18C for 96% of the year. On this basis, therefore, chilled ceiling panels and beams could be supplied, in Irish climatic conditions, for approximately 7,000 hours per annum. In the London area, on the basis of the Heathrow data (1949-1976) presented by De Saulles (3), it wou

Sensitivity Studies of a Low Temperature Low Approach Direct Cooling Tower for Building Radiant Cooling Systems

Recent interest in cooling towers as a mechanism for producing chilled water, together with the evolution of radiant cooling, have prompted a review of evaporative cooling in temperate maritime climates. The thermal efficiency of such systems is a key parameter, as a measure of the degree to which the system has succeeded in exploiting the cooling potential of the ambient air. The feasibility of this concept depends largely however, on achieving low approach water temperatures within an appropriate cooling tower, at acceptable levels of energy performance. Previous experimental work for a full scale evaporative cooling system has shown that it is possible to produce cooling water at low process approach conditions (1-3 K), at the higher temperatures required in radiant and displacement systems (14-18 °C), with varying levels of annual availability in different temperate climate locations. For such conditions, evaporative cooling has the potential to offer an alternative approach for producing chilled water, particularly in temperate climates, where conventional mechanical air-conditioning systems can, for certain buildings, be considered to be an over engineered solution but where passive cooling is insufficient to offset cooling loads. The current paper describes the development of a mathematical model which analyses the behavior of a low approach open evaporative cooling tower. The model is used to carry out a series of sensitivity studies assessing the performance of the cooling tower subject to various weather and climatic boundary conditions.

Thermal Efficiency Characteristics of Indirect Evaporative Cooling Systems

Recent developments in enhancing heat transfer in cooling towers, together with the success of chilled ceilings, have prompted a review of the evaporative cooling technique. in temperate maritime climates. The thermal efficiency of such systems is a key parameter, as a measure of the degree to which the system has succeeded in exploiting the cooling potential of the ambient air. This paper presents the results of experimental research into the thermal efficiency of a water-side open indirect evaporative cooling test rig designed to achieve low (1-4 K) approach conditions. Secondary efficiencies in the range 0.24-0.76 have been achieved.

Energy Reduction by Enhanced Evaporative Cooling of Buildings in Maritime Climates

Direct cooling of buildings by water evaporation has traditionally been seen as appropriate, only, in dry and arid climates, which experience high levels of wet bulb temperature depression. The technique has generally not been applied in maritime climates where low levels of wet bulb temperature depression are frequently found. However, recent developments in enhancing heat and mass transfer in cooling towers, together with the success of high temperature sensible cooling systems, such as chilled ceiling panels and beams, have prompted a review of the evaporative cooling technique as an effective and low energy means of cooling modern deep plan buildings, in maritime climates. At present, however, there is little in depth research and analysis of the performance, energy efficiency, and availability of this form of cooling in maritime conditions. To address these issues an experimental research programme has been established with a view to demonstrating the potential and optimising the design of this form of cooling under low approach conditions. This paper presents the results of recent experimental research into the electrical consumption of a prototype inverter controlled cooling tower when generating cooling water at the chilled water temperatures required for chilled ceiling panels and beams, under varying load and wet bulb temperature approach conditions. Energy consumption efficiencies are presented for a range of specific conditions and typical annual efficiencies are computed. Results are compared with typical energy consumption efficiencies of conventional, vapour compression based, cooling systems. A considerable potential for the reduction of cooling electrical energy consumption, in maritime climates, is shown.

Mathematical Modelling of a Low Approach Evaporative Cooling Process for Space Cooling in Buildings

This paper describes a mathematical model of a low approach open evaporative cooling tower for the production of high temperature indirect cooling water (14-16C) for use in building radiant cooling and displacement ventilation systems. There are several potential approaches to model evaporative cooling, including: the Poppe method, the Merkel method and the effectiveness-NTU (e-NTU) method. A common assumption, applied to the Merkel and e-NTU methods, is that the effect of change in tower water mass flow rate due to evaporation is ignored, which results in a simpler model with reduced computational requirements, but with somewhat decreased accuracy. In this paper, a new improved method, called the corrected e-NTU approach is proposed, where the water loss due to evaporation is taken into account. It is expected by this correction the results of improved e-NTU in the category of heat transfer will be more close to the results of more rigorous Poppe method. The current mathematical model is evaluated against experimental data reported for a number of open tower configurations, subject to different water temperature and ambient boundary conditions. It is shown that the discrepancies between the calculated and experimental tower outlet temperatures are to within ±0.35C for a low temperature cooling water process (14-16C), subject to temperate climate ambient conditions and ±0.85C for a high temperature cooling water process (29-36C), subject to continental climate ambient conditions. Considering the associated tower cooling loads, predicted results were found to be within a 6% root-mean-square difference compared to experimental data.