COGENERATION AND THE SUGAR INDUSTRY
Potential for cogeneration exists in the sugar industry where steam is raised from bagasse, a waste product of the cane milling process, for power generation and process heat. Unlike other industries that only consume energy, the sugar industry can generate surplus power over and above its internal requirements by burning bagasse to generate process steam and power. However, due to statutory requirements and other limitations on the sale of electricity, sugar factories in Kenya have been unable to exploit energy in bagasse. Excess bagasse is currently treated as waste and incinerated, largely as a process of disposal. Process steam and power are in this case unfortunately treated as a by-process of the disposal exercise.
The current Kenyan government, voted in on a platform of accelerated economic development and infrastructure rehabilitation, has committed itself to, among other initiatives, the rehabilitation of the sugar industry. Power generation through cogeneration is seen as opening up new avenues for revenue creation in the sector. Accordingly, the Ministry of Energy recently permitted sugar companies to generate power for sale to the national grid and to the public in general. Furthermore, various fiscal incentives for investments in regular and non-conventional renewable energy projects have been suggested for inclusion in the national energy policy document. Meanwhile, local utilities are looking at strengthening the transmission grid, which, coincidentally, will allow the sugar companies to feed in their power. Consequently, it has become a viable proposition for sugar companies to raise high-pressure steam in modern, high-efficiency boilers using bagasse to generate heat and power economically to provide surplus power for export to the grid.
Like other sectors in the Kenyan economy, the sugar industry has undergone continuous decline over the past decade. Statistics provided by the Kenya Sugar Board indicate that the best performance in the last decade (1992-2002) was recorded in 1996 when the industry had 131,100 hectares under cane yielding a healthy 90.9 tonnes of cane per hectare and supporting 23,900 plantation jobs in direct wage employment. In contrast, 2002 figures show that the acreage had dropped to 126,800 hectares, while the yield was down to 70.7 tonnes per hectare with the number of plantation jobs down to 2630.
Enormous potential in the Middle East and China
District cooling is a well-established technology in the US. Over 30 new systems have been developed in US city centres since 1990 and significant expansion is underway on educational and other campuses, usually as part of a wider district energy scheme. Now, the technology is starting to take off in the Middle East and China. Robert Thornton outlines the US experience and surveys the wider potential.
The fundamental operating principle for district cooling (DC) involves providing chilled water from a central plant to multiple buildings via an underground network of chilled water supply and return pipes. The interconnection of multiple buildings for air conditioning service creates an economy of scale and diversity of load shapes that allows more efficient use of capital, better load factors and operating efficiency of equipment, and environmentally-optimum energy sources. These include combined heat and power (CHP), industrial waste heat, thermal and ice storage, and also renewable energy sources such as biomass, geothermal and natural sources of heating and cooling.
District cooling systems supply ‘ready-to-use’ chilled water, which typically is pumped to the building at 3º-6ºC with some positive pressure differential. The chilled water flows either directly through the building HVAC system, or is interconnected via heat exchanger(s), especially in high-rise buildings for pressure separation. The chilled water absorbs heat from the space via the building air handlers, warming up by 6º-11ºC, and then flows back to the central plant in a separate return loop where it is reconditioned and redistributed. Figure 1 shows one building connection arrangement.
Figure 1. Typical building connection method
The alternative – conventional on-site cooling systems – typically require electrically-driven equipment on, in or near the building. This increases the building’s electricity demand during peak electricity pricing hours when air conditioning is needed most.
There are numerous advantages to district cooling service. Building owners reduce the capital costs for construction by eliminating on-site chillers and cooling towers, and by reducing ancillary equipment such as electrical vaults, pumps and chemical storage. Valuable rentable space is freed up in the customer building, especially penthouses and rooftops. Noise, vibration and on-site chemicals (water treatment, refrigerants, etc.) are also eliminated or reduced, along with expenses for labour, maintenance and operations.