True accounting

High network costs make DE the better alternative

In a quiet Paris street, close to the River Seine and tucked away at the back of the Australian Embassy, is the office of the economic and energy modellers of the International Energy Agency (IEA). Their goal is to predict the world’s energy future, and their rojections are among the most influential in the world. Their World Energy Model is a powerful tool used by policymakers and the energy industry everywhere. The most recent modelling work includes for the first time some practical consideration of decentralized energy and CHP. This is just one example of the Model’s continuous improvement and it is a good start. But there is still more scope for the Model to reflect current commercial and economic reality in its assessment of DE. More of that in a moment.

The latest volume of analysis and outputs is the IEA’s World Energy Outlook 2004 (WEO).1 It is a treasure of data, statistics and projections – terrific reading. The IEA modellers have used two scenarios in its projections for the 2004 Outlook. The first, the Reference Scenario, assumes no change in current energy and environmental policies (and therefore includes continuing dominance of central power in the power generation sector). Some of the highlights make sobering reading:

  • electricity demand doubling by 2030
  • gas use overtaking that of coal by 2015 and doubling by 2030
  • developing countries accounting for two thirds of demand growth
  • carbon emissions almost doubling by 2030 compared with 1990, the Kyoto Protocol base year.

The environmental and supply challenges facing the energy sector in 2005 look bad enough. On the basis of the Reference Scenario, we are in for a bleak future. Fortunately, there is little chance that this will come to pass. Policies are dynamic, not static, and in most countries we have already seen the gradual emergence of initiatives to improve efficiency and cut the carbon intensity of energy generation and use. Far too slow, but it is a start. This will not only continue, but will certainly accelerate. Hence, a new and alternative scenario of the IEA, neatly called the Alternative Scenario, makes projections on the basis of more sustainable policies that are currently under consideration or which could be expected over the next few years. Even here, carbon emissions increase 30% by 2030, but at least this scenario presents a much more plausible view of the future.

for CHP plants

The design of heat recovery steam generators for use in cogeneration plants presents a series of challenges. Pat Albert takes a look at options for heat transfer surfaces, duct liners and fresh air firing.

Heat recovery steam generators (HRSGs) recover energy from gas turbine exhaust to produce steam. When the HRSG is part of a combined-cycle system, the steam conditions are defined only by the steam turbine requirements. When used in a CHP application, it is the process requirements that define the amount of steam needed, as well as the steam pressure and steam temperature. When steam flow is critical to the process, redundancy in the HRSG design is also specified in the form of multiple HRSGs and/or fresh air firing capability.

Because the requirements at every plant or site are unique to that site, each application is unique and requires a different HRSG design. For CHP applications, it is not unusual to find steam conditions ranging from 100 psig (690 kPa) saturated steam to over 1200 psig/980oF (8270 kPa/527oC) superheated steam. As well as different steam conditions, process or permit requirements may demand the addition of a duct burner or emissions control equipment.

In order to obtain an HRSG system at the lowest cost, it is essential to specify the performance requirements

In order to respond to the many different requirements, HRSG designers need to ensure boiler components have the appropriate amount of surface area and to incorporate a variety of auxiliary equipment into the system. Auxiliaries could include burners, dampers, pumps, fans, de-aerators, CO catalysts, selective catalytic reduction (SCR) catalysts, etc.

For the customer to obtain an HRSG system that meets their needs for the lowest cost, it is essential to specify the performance requirements. Because processes differ from plant to plant, it is impossible to define all possible requirements, but some are common for all projects. These are listed below: