REMANENTE SEPARADO 3. Gloria Postrera

The 3P perspective has previously been used to show energy consumption in a manufacturing environment via framework to model Embodied Product Energy (Rahimifard et al., 2010), which

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gives a clear indication of where energy consumption “hotspots” exist within manufacturing process chains, and is used to gauge whether energy is used efficiently. Similar to the 3P perspective, the International Standards Organisation (ISO) has developed a model composed of a six-level hierarchical structure (Figure 8.2), including enterprise, facility, machine cell, machine and turret. The enterprise level is responsible for the achievement of the mission of the enterprise and clearly its planning horizon is measured in the long term. The facility or plant level is responsible for the implementation of the enterprise functions and the reporting of status information to the enterprise level. It includes functions such as manufacturing engineering, information management, production management and scheduling and production engineering. At the machine cell level, the responsibilities are provision and allocation of resources and the coordination of production on the shop floor. The cell level is responsible for the sequencing of jobs through the various stations. Its functions include resource analysis and assignment, making decisions on job routings, dispatching jobs to individual machines and the monitoring of task and machine status. The turret level is responsible for the direction and coordination of relatively small integrated workstations (Bauer & Browne, 1994). When considering the theoretical degree of detail at the turret level and diverse range of activities at the enterprise level, which may present significant problems with the availability of energy data, these two manufacturing levels would likely prove problematic. Therefore, in this research, only the facility, machine cell and process level are considered, highlighted by red circles in Figure 8.2.

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In this research the 3P perspective is used to identify where waste heat is available for recovery in a manufacturing environment. This is because not only are the energy survey requirements rather different for these three perspectives but also the quantity and quality of energy available is significantly different and so will be the requirement of technology for energy recovery. A diagram of the 3P perspective and how they interact with each other has been created shown in Figure 8.3. In general, the applicability for waste heat recovery is to reuse the energy for the same activity or to cascade its use to an activity at a higher manufacturing level. In order to develop a detailed breakdown of waste heat sources within a production facility, there is a need to systematically identify waste heat hot spots and this leads to a complete survey of the facility.

8.2.1 Plant

From a ‘plant’ level perspective, heat energy is required by the infrastructure and other high level services that are responsible for maintaining the required production conditions/environment. Examples of ‘plant’ level heat usage are lighting, space heating, air-conditioning and ventilation. Heat is lost from a broad range of activities in a manufacturing facility, and this is contributed by building heating, cooling, and ventilation sections of the facility.

Figure 8. 3 Red arrows represent waste heat being generated; green arrows indicate recovered waste heat energy and blue arrows show potential for reuse within the same activity

e.g. Hot product leaving e.g. excess steam from

turbines and boilers

e.g. Hot gas from exhaust and flue stacks

WHE

Plant

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Examples of such activities include boilers which provide space heating or hot water, ventilation/air conditioning systems without any means of recirculation or heat recovery, water chiller or other cooling plant rejecting heat to outside, and temperature control in high spaces. Demand for these activities may vary significantly depending on the weather and season of the year. For example, boilers are required to work throughout winter to provide space heating and hot water to the building but space heating may not be necessary during hot summer days. Ventilation/air conditioning system may be required to be fully operational throughout the year to maintain a suitable working environment.

At the plant level, much of the heat loss improvements are encompassed as part of a generic Energy Management System (EMS) which would typically involve energy audits and monitoring. However the investment required for recovering unutilised heat at this level could be very significant, for example the major cost of restructuring of the heat systems to the factory.

8.2.2 Process

In a manufacturing facility all energy used by a process, that does not reside in the products as it leaves the process is ultimately lost to the environment. Some of the energy is useful, e.g. heat from coolant pumps, and some less useful, e.g. lighting an unmanned process. From this perspective, the existing research typically has investigated areas for energy improvement which relates either to the improvement of operational procedures or machine design. This improvement in operational procedures could include minimising idle time in a process through better production planning or improving the process through more effective sensing and control. The improvement in machine design could include elimination/reduction of non-essential activities (e.g. coolant, lubricant). These improvements are of particular interest to manufacturers especially where energy savings make economic sense. However such investments are only capable of reducing energy demand so far before they become financially prohibitive. Therefore a reactive approach to recovery of waste heat energy that could potentially be more cost effective should be considered.

A study done by the U.S. Department of Energy reported that a significant percentage (20-50%) of industrial energy inputs is lost as waste heat in the process stage, totalling anywhere from 2 to 4 TWh (Johnson et al., 2008). This highlights that there is a significant amount of waste heat can be recovered, and also a need for energy transparency across a production facility so that energy hotspots can be identified.

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8.2.3 Product

The integration of energy considerations at the plant and process perspective into a ‘product’ perspective would give manufacturers effective indication of which activities and processes are energy intensive and/or potential waste heat hotspots. Seow & Rahimifard (2011) used the ‘product’ perspective to systematically identify the total energy required to manufacture a product, also referred to as the Embodied Product Energy (Rahimifard et al., 2010). The study undertook an in- depth analysis of energy requirements based on product design features, manufacturing parameters and operational procedures. The study aimed at enabling engineers and designers to identify the energy hotspots during production and facilitate process design optimisation, production and process planning improvements as well as product design enhancements.

In this research, waste heat energy at ‘product’ level is defined as the heat contained in the final products when it leaves its treatment process. It is present in the form of heat energy in products coming out of the process, and they may still carry a significant amount of residual heat from their treatment, for example, in metal casting or oven baking process. This heat is unwanted and dispersed using means of cooling, e.g., water or air. Some products may be required to, or will naturally cool at particular rate depending on desired final perspectives, etc., different specific heat capacities.

As discussed in Chapter 4 of this thesis, heat at the product level of manufacturing has not been paid much attention and is often wasted despite being potentially recoverable. Although the waste heat temperature of the products, especially with metal casting products, can be are reasonably high, the waste stream continuity is not constant and there are issues with other waste heat quality criteria which will be discussed in later sections.