A BSRIA Guide. Free Cooling Systems BG 8/2004 FREE COOLING SYSTEMS BSRIA By Tom De Saulles. Supported by

Transcription

1 A BSRIA Guide BG 8/2004 FREE COOLING SYSTEMS BSRIA 2004 Free Cooling Systems By Tom De Saulles Supported by BG 8/2004

2 CONTENTS 1 INTRODUCTION 1 2 WATER-SIDE SYSTEMS Indirect water-side systems Load shaving Direct water-side systems Performance issues Payback for a water-side free cooling system Thermosyphon systems 40 3 AIR-SIDE SYSTEMS Introduction Recirculation systems Full fresh air systems 58 4 MISCELLANEOUS SYSTEMS Direct groundwater cooling Fan coils Chilled ceilings and beams Dry coolers Free cooling chillers Evaporative pre-coolers Evaporative coolers 72 5 KEY SYSTEM ATTRIBUTES 74 6 CASE STUDIES 75 GLOSSARY 90 REFERENCES 91 APPENDICES APPENDIX A: Enthalpy and moisture content, air-side control strategy 88 TABLES Table 1: Results of energy saving calculation 38 Table 2: Results of financial savings and payback calculation 39 Table 3: An energy saving example of a free-cooling chiller 70 Table 4: List of case studies 75 Table 5: Annualised system performance 80 Table 6: Life-cycle costing calculation for the data centre 81 Table 7: Control logic sequences 89 FREE COOLING SYSTEMS

3 FIGURES FIGURES Figure 1: Categories of free cooling systems 2 Figure 2: Main types of water-side free cooling (blue line denotes free cooling flow) 5 Figure 3: Basic configuration of an indirect tower-based free cooling system 6 Figure 4: General control strategy for indirect tower-based free cooling 9 Figure 5: Exploded view of a typical plate heat exchanger showing the path of flow on hot and cold sides (supporting frame not shown for clarity) 10 Figure 6: Relationship between approach and area/cost for a typical plate heat exchanger 12 Figure 7: Relationship between additional savings and plate heat exchanger cost for approach temperatures below 3 K 12 Figure 8: Relationship between flow and cost for a typical plate heat exchanger 12 Figure 9: Use of eccentric reducers to improve liquid-solid separation in side stream filtration 15 Figure 10: Load shaving system using existing cooling towers 17 Figure 11: Basic configuration of a direct tower-based free cooling system 18 Figure 12: Direct free cooling using a common condenser water header 20 Figure 13: Direct free cooling using a dedicated tower and chiller arrangement 20 Figure 14: Automatic back-flushing single-element filters 22 Figure 15: Internal view an automatic back-flushing singleelement filter 23 Figure 16: Configuration of hydrocyclone separator to reclaim water used for flushing 24 Figure 17: Example of two installed variable-gap filters 25 Figure 18: Variable gap filter screen depicted in the filtration mode on the left, and back-flushing mode on the right 25 Figure 19: Annual hours the ambient wet bulb temperature is at or below a given value (bassed on data for heathrow airport ) 28 Figure 20: Monthly average wet bulb temperatures for London, Manchester and Edinburgh ( ) 28 Figure 21: Free cooling availability at varying wet bulb temperatures (Heathrow) 30 Figure 22: Optimal chilled water temperature control diagram (for a constant volume system) 33 Figure 23: Approach of a typical cooling tower operating at varying loads and wet bulb temperatures 34 Figure 24: Cooling tower over-sizing margin at varying loads and values of approach 35 Figure 25: A basic thermosyphon chiller arrangement (arrows indicate flow in thermosyphon mode) 41 Figure 26: Hysteresis effect at start of the thermosyphon process 42 Figure 27: Boiling at the enhanced surface of evaporator tubes 42 Figure 28: A multiple chiller thermosyphon system 44 Figure 29: The three port change over valve in an ammonia based system 44 FREE COOLING SYSTEMS 03/11/04 FREE COOLING Systems

4 FIGURES Figure 30: Single chiller Thermosyphon system with a thermal buffer (arrows indicate flow in thermosyphon mode) 46 Figure 31: Modulating damper system without humidity control 50 Figure 32: Damper operation within an ambient psychrometric envelope 51 Figure 33: Modulating damper system operating under dewpoint control 54 Figure 34: Dewpoint control summer 54 Figure 35: Dewpoint control (spring/autumn) 55 Figure 36: Dewpoint control (winter) 55 Figure 37: Modulating damper system operating under moisture content and enthalpy control 56 Figure 38: Enthalpy and moisture content control 56 Figure 39: Moisture content control 56 Figure 40: Static pressure drop (constant pressure operation) 57 Figure 41: Full fresh air system with air-to-air plate heat exchanger and indirect evaporative cooling 60 Figure 42: Psychrometric process for system depicted in figure Figure 43: Sprayed plate cooler in operation 61 Figure 44: Desiccant cooling system incorporating thermal wheel and evaporative humidification in supply and extract 62 Figure 45: Psychrometric process for a desiccant cooling system depicted in figure Figure 46: A central air handling plant cooling coil used to provide free cooling in a fan coil system 65 Figure 47: Load shaving with a dry cooler 67 Figure 48: Simplified free-cooling chiller, with the free-cooling coil operational 68 Figure 49: Typical free-cooling chiller operation at varying ambient temperatures (based on weather data for London) 69 Figure 50: Example of a free cooling chiller 69 Figure 51: Typical application of evaporative pre-coolers 71 Figure 52: Terminal 1, Heathrow Airport 76 Figure 53: One of the induced-draught cooling towers 77 Figure 54: British Airways Data Centre, Cranebank, Heathrow 78 Figure 55: Two views of one of the thermosyphon chillers installed at the data centre 78 Figure 56: Condenser and chiller capacities for a range of dry bulb and chilled water temperatures at the data centre 79 Figure 57: Portcullis House, Westminster 82 Figure 58: The borehole cap at Portcullis House 83 Figure 59: Simple basket strainer used to filter the groundwater water 84 Figure 60: PHEs exchange heat between the groundwater and chilled water circuit 84 Figure 61: Learning Centre - Main Entrance Interior, Leeds Metropolitan 86 Figure 62: The spray nozzles on the underside of the evaporative coolers at the Leeds Metropolitan University Learning Centre 87 Figure 63: Control logic flow diagram 88 FREE COOLING SYSTEMS

5 INTRODUCTION INTRODUCTION INTRODUCTION The ongoing drive to reduce the environmental impact of the UK building stock has increased the pressure on architects, engineers and building operators to avoid the use of air conditioning in favour of more passive cooling solutions - such as natural ventilation and the use of thermal mass. Good progress has been made in this direction, but many passive solutions are limited to new build projects, which only represent 2-3% of the UK building stock. For existing buildings, and those for which mechanical air conditioning cannot be avoided, other options must be considered. Unfortunately, mechanical air conditioning is often perceived as a system for which only very limited energy saving measures can be applied. This is rarely the case, as the incorporation of a free cooling capability can significantly improve overall efficiency. Free cooling in the context of this Guide can be broadly defined as that amount of cooling which can be obtained to wholly or partly offset the load on mechanical refrigeration plant. With the exception of two systems, the techniques covered in this guide satisfy this definition. The exceptions are ground water and desiccant cooling, which have nevertheless been included for their relevance. Free cooling systems take advantage of favourable weather conditions to enable chiller plant to be shut down for long periods, saving significant amounts of energy and cutting carbon emissions. The technology associated with free cooling is neither complex or new, but opportunities to make use of it are often overlooked or not understood. The purpose of this Guide is to help address the problem by providing free cooling design and application information for designers, technically-aware clients and building operators. The basic free cooling techniques detailed in this Guide each have particular attributes suited to many different types of air conditioning systems. This makes free cooling a very versatile energy saving measure that is not limited to new buildings. The business case for retrofitting free cooling is often quite compelling, and the payback period can be short. Disruption to business activities during installation is often minimal, as most of the plant can often be installed alongside existing plant and interconnected at the end of the installation process. Due to the bespoke nature of free cooling systems, no single guide can anticipate the circumstances and demands of all applications. However, this Guide is intended to provide a general insight into the main free cooling techniques that can be applied. The content has not been divided between new build and retrofit projects as much of the guidance is applicable to both, although where this is of particular relevance the distinction is made. The report provides an understanding of the types of system and their associated advantages and limitations, the requirements for effective application, the methods of system control, and general guidance on system design and sizing of components. FREE COOLING SYSTEMS 1

6 1 INTRODUCTION Figure 1 shows the various free cooling systems included in the Guide, and the three ways in which they have been categorised. The main characteristics of each system are provided in a summary table located at the end of the Guide (see Table 7). Figure 1: Categories of free cooling systems. Important requirements for effective free cooling Selecting a water-side free cooling technique appropriate to a particular project is relatively straightforward, as it is largely dictated by the type of installed or proposed air conditioning system. Assessing the likely energy saving potential is a little more involved. However, there are several essential questions which are relatively easy to answer, and will provide an indication of whether it is worth continuing with a more detailed assessment. These questions are summarised below, and given more detailed coverage in Section 2, which concludes with a worked example of a payback calculation. The following questions should be answered to determine whether a water-side free cooling system may be effective: Is there a significant cooling load during the winter months, for instance greater than 20% of the design full cooling load? Is the installed capacity of the chiller plant greater than approximately 1 MW? Is the winter cooling load present on a 24h basis? 2 FREE COOLING SYSTEMS

7 INTRODUCTION 1 Is a relatively high chilled-water temperature acceptable? Is there spare cooling tower capacity such as a standby tower, and/or are the tower(s) known to be oversized or more than adequate to meet the peak summer load? If the answers to these questions reveals a potential for free cooling, it will be necessary to obtain detailed load profile data and performance information for the towers in order to make a more detailed assessment (see Section 2.4, page 28). Airside systems must be addressed in a different manner, as the majority of devices and techniques that can be used to provide free cooling in airside systems can also provide heat recovery in the winter. In fact, it is the heat recovery capability that is likely to save the most energy and determine the approach used. In modern design practice it is very unlikely that an air-side system would be designed without some form of free cooling or heat recovery capability. As it is standard practice to incorporate some form of heat recovery/free cooling capability into airside systems, the key design issue is not whether to incorporate such a capability, but rather which technique to use. This question is examined in Section 3. FREE COOLING SYSTEMS 3

8 2 WATER-SIDE SYSTEMS WATER-SIDE SYSTEMS 2 2 WATER-SIDE SYSTEMS The term water-side system in the context of free cooling systems describes free cooling techniques that can be applied in centralised chilled water installations incorporating cooling towers or evaporative coolers (see Figure 2). Water-side systems can be divided into two types: direct and indirect. Direct water-side systems (Section 2.3) physically interconnect the chilled water and condenser water circuits during free cooling operation, enabling heat to be rejected directly by the cooling towers without the need to run the chillers. The main benefit of this technique is that the chilled water to ambient wet-bulb approach temperature is kept to a minimum, which in turn maximises free cooling availability. The drawback of direct interconnection is the need for a strict water treatment regime to reduce the risk of corrosion and prevent dirty water from the cooling tower fouling the chilled water circuit. This risk has led to a decline in the application of direct systems. Although briefly popular in the 1980 s, direct systems have since been largely overshadowed by indirect systems. A variation on the water-side technique described above is the loadshaving system (Section 2.2). This makes use of one or more cooling towers or evaporative coolers to pre-cool the return chilled water before it reaches the chiller(s), and consequently reduces the mechanical cooling load. This technique ensures that, even during marginal ambient conditions, some free cooling (load shaving) is possible, which is not the case for direct and indirect water-side systems. Under favourable ambient conditions some load shaving systems can also provide full free cooling, enabling the chiller(s) to be shut down. An example of a load shaving system is described later (see Section 2.2). Indirect water-side systems (Section 2.1) avoid the problem of fouling by maintaining the chilled water circuit as a closed loop and rejecting heat to the cooling towers via a plate heat exchanger (plate heat exchanger). Alternatively, an indirect system may incorporate one or more closed circuit cooling towers or evaporative coolers. The disadvantage of indirect systems is the introduction of an additional heat transfer surface, which increase the chilled water to ambient wet-bulb approach temperature, resulting in a slight reduction in free cooling availability. Thermosyphon systems (Section 2.6) can also be described as an indirect water-side system, as the chilled water circuit remains hydraulically isolated at all times. This technique uses the difference in temperature/pressure between the evaporator and condenser during cooler weather to drive refrigerant around the circuit without the need to operate the compressor. Thermosyphon systems can be particularly effective as they offer both a free cooling and load shaving capability. 4 FREE COOLING SYSTEMS

9 WATER-SIDE SYSTEMS 2 Figure 2: Main types of water-side free cooling (blue line denotes free cooling flow). 2.1 INDIRECT WATER- SIDE SYSTEMS Case study 1 As the name suggests, indirect tower-based free cooling systems reject heat to cooling towers indirectly, typically by means of a plate heat exchanger (see Figure 3). Consequently, as the condenser and chilled water circuits are at no point interlinked during free cooling, the risk of increased fouling and corrosion associated with direct systems is avoided (see Section 2.3, Direct water-side systems). The penalty for using a plate heat exchanger is an increase in the chilled water to wet-bulb approach temperature, the size of which will cause a corresponding reduction in free cooling availability. With very few exceptions, the trade-off is generally judged to be worthwhile, as the level of maintenance and risk of fouling associated with direct systems may be regarded as too onerous. The evidence for this can be seen in the increasing number of indirect systems specified in recent years, and the almost complete absence of direct systems, which had their heyday in the early to mid-1980 s. FREE COOLING SYSTEMS 5

10 CASE STUDIES CASE STUDIES CASE STUDIES Five case studies are presented, two dealing with free cooling in waterside systems and three using other cooling techniques and equipment. Table 4: List of case studies. 1. Indirect water-side system Terminal 1, Heathrow Airport 2. Thermosyphon system British Airways Data Centre, Heathrow 3. Direct ground water cooling Portcullis House, Bridge Street, Westminster 4. Chilled ceiling Department of Trade and Industry, London 5. Evaporative cooler City Campus Learning Centre, Leeds Metropolitan University FREE COOLING SYSTEMS 75

11 4 CASE CASE STUDY STUDIES 1 Indirect water-side system for Terminal 1, Heathrow Airport Figure 52: Terminal 1, Heathrow Airport. Background Due to the increase in retail space within the Terminal 1 over the past 20 years the cooling requirements at the Terminal have changed. The cooling demand profile now means that parts of the Terminal need to be air-conditioned throughout the year. An initial feasibility study was undertaken to identify the potential for adopting a free cooling system in order to reduce costs. Power Plan Services undertook this study and the subsequent free cooling design, which was developed to coincide with the installation of a third chiller. This meant that all of the pipework modifications and connections were made such that the free cooling system could be installed with the minimum of disruption. When the cooling towers were replaced in1999, it was decided to incorporate a free cooling system. The system comprises a stainless steel plate heat exchanger adjacent to one of the chillers. The water which passes over the cooling towers is usually at 25 C, but when the outside temperature is low enough the towers can produce water at 6 C. Heat is transferred between the cooling towers and the chilled water via the plate heat exchanger. This means that the chiller, which is designed to produce water at 6 C, is replaced by the plate heat exchanger. The system functions by a series of large valves redirecting the cooling tower water and chilled water through the plate heat exchanger. The control system is linked to the BMS, which continuously monitors the outside air temperature and moisture content. When the conditions are suitable the changeover to free cooling is fully automatic. 76 FREE COOLING SYSTEMS