Cooling options for emergency backup diesel generators14 January 2014
The choice of air or water as cooling medium for additional emergency backup diesel generators has profound design and process implications for the site. By Nader Ben Said
The provision of additional diverse stationary diesel generators dedicated to the management of severe accidents has been in discussion in many nuclear power plants worldwide as a result of the Fukushima accident. These stationary additional emergency AC power sources are designed to assure ultimate power supply for maintaining safety-related functions during and after external events resulting in loss of emergency power supply.
The associated diesel generator sets and related instrumentation and control (I&C) equipment as well as electrical equipment need cooling for proper functioning. This article aims to provide a generic comparison of cooling options for diverse stationary diesel generators (namely water and air cooling) and experience-based recommendations for approaching related new projects.
At the beginning of an additional diverse diesel generator installation project an extensive external events analysis needs to be carried out to properly define the site-specific risks (for example flooding, earthquake, tornado, shock wave, man-made hazards, extreme ambient temperatures).
We consider an additional diesel generator with related systems and equipment with an electrical power of 4 MWe. The simplified scheme shown in Figure 1 gives an overview of the configuration related to an additional diesel generator.
The main cooling requirements for an additional diesel generator are:
- ˜ 3500 kW heat capacity for charge-air and motor cooling
- ˜ 300 kW dissipated heat from engine and generator in DG-set room
- ˜ 5 kW dissipated heat from instrumentation & control equipment
- ˜ 30 kW dissipated heat from electrical equipment
In general, heating, ventilation and air conditioning systems are used to cool the diesel generator-set room, and the I&C as well as electrical equipment. Furthermore, all needed equipment is typically integrated into a building designed against external events.
It is emphasized that in this article strictly diverse systems (to the nominal emergency power supply systems) are considered. That means specifically that the cooling option should be independent from the nominal ultimate heat sink (UHS) of the plant.
Water and air cooling
The configuration of water cooling in its simplest form is shown in Figure 2. Three typical main process parameters for water cooling are these: a water inlet temperature of 30°C (assumption for stored water), water discharge temperature of 80°C (limited by engine temperature), and required water flow of 66 cubic metres per hour. To give an engineering view, the following illustration is given. If the diverse heat sink is designed as stored water, a reservoir with a capacity of about 11,000 m3 is needed for one week's operation.
The configuration of air cooling in its simplest form is shown in Figure 3. Typical main process parameters for air cooling are these: an air inlet temperature of 45°C (assumption for ambient temperature under extreme weather conditions), air outlet temperature of 80°C (limited due to engine temperature), and a required air flow of ˜ 400,000 cubic metres per hour.
How much space is needed?
Designing the air inlet and outlet limitations on the air velocity are to be taken into consideration. If the limit of 2 m/s for air velocity at inlet and outlet is taken as a design base, an area of about 55 m2 is needed respectively for air inlet and air outlet. Inlet and outlet area can significantly impact the design and dimensions of the building.
To give an engineering view of the required space (building dimensions), a simplified illustration is given in Figure 4. The white block represents the size of building for all the equipment related to an additional diverse stationary diesel generator except the cooling equipment. The space needed for the equipment related to water cooling is relatively small. In contrast, for air cooling a building with a volume roughly 150% larger is needed.
A typical water cooling system is relatively simple (mechanically and in terms of I&C), has few components, and its construction has practically no impact on civil work and building design. These features impact its costs. However, a diverse water supply (independent from UHS) is required, and its related costs shall be taken into account where applicable. On the other hand, an air cooling system is rather complex (mechanically and in terms of I&C), has components numbering in two digits, and its construction has a large impact on civil work and building design, in terms of required space, physical protection for building openings, and so on.
Below are two experience-based recommendations for initiating a diverse stationary diesel generator retrofit project. First, evaluation and assessment of appropriate options for cooling should be done in a pre-concept phase of the project. The assessment should be based on the specific conditions of the plant (site requirements, availability of a diverse water supply). Second, in case of air cooling, a (pre-)concept design for cooling (preliminary dimensioning) should be performed in the pre-project phase and considered in finalizing the building design.
In case air cooling is favoured, the following points should be considered:
- The required space for air cooling is approximately one-third the volume of the whole building
- The building should not be too small. This could lead to complex technical solutions, considerably higher engineering costs and may significantly impact schedule.
- Openings for air inlets and outlets need to be properly sized and positioned (complying with weather protection requirements, optimizing power consumption/engine size/fuel storage, protection against external events). This could impact building design.
- Front-end technical iterations between engineering activities are the best risk-mitigation strategy for such projects. There is a very solid experience base in the industry to implement emergency diesel generator projects for an optimal solution for each site/plant.
Nadir Ben Said is group leader, plant engineering, Westinghouse Electric Germany.
This article is based on a paper presented at the Tuev symposium on emergency power systems at nucelar power plants in Munich, Germany, 11-12 April 2013.