Retrofit : Retrofit Volume 1 2012
30 • RETROFIT AUSTRALIA • VOLUME 1 NUMBER 1 2012 | Energy Efficiency needs more space for plant, pumps and cooling tower usage. Even if used for cooling, the heat from the building and the generation needs to be rejected to atmosphere. If energy efficiency measures have reduced the building cooling load (for example by upgrading the façade), then perhaps your existing cooling towers will be sufficient; however, it is a good idea to get this checked by a mechanical engineer, as cogeneration systems can require 1.4 times normal cooling tower size requirements. In terms of plant room equipment, a gas engine is typically larger than a diesel standby generator, and an absorption chiller is also larger and heavier than an electric chiller. A large 2.25-megawatt cogeneration installation may need 540 square metres for the switchboards, multiple generators (three), an absorption chiller, control room, pumps, fans and pipework. A small 50-kilowatt cogeneration unit, however, can be under 1.5 metres wide and under 3.5 metres long, so solutions can typically be found but may require some ingenuity. If you do need power generation for increased reliability, bear in mind that gas engines need to be off for maintenance on average at least two weeks per year (usually about one week in one go, but if you need a new part this can take more time, just like your car service) with regular oil changes and filter changes, depending on run hours. Also, five- yearly and 10-yearly services are needed, and are longer and more costly. This means that the electricity grid will more than likely be a more reliable energy source, and you may need to retain most – if not all – of your existing plant. Diesel generators are still likely to be needed, as they respond faster and quicker than a gas engine. You can transfer load from a diesel engine to a gas engine for long power outages, but it is hard to remove the diesel generator without losing reliability or requiring an excessively large uninterruptable power system (UPS). Sizing the plant is probably the most difficult part of the design, with the following three items to be closely matched: • heating requirement (daily variation and yearly variation) • cooling requirement (daily variation and yearly variation) • power usage (daily variation and yearly variation). Each of the above options needs to be considered and usually modelled. A gas engine does not perform very well below 40 per cent of the peak capacity, which can lead to some cogeneration units never operating due to excessive maintenance and operational costs. Sizing on the smallest base energy constant load requirement (for example, base building electricity usage or the yearly domestic hot water demand) can be the safest option, but can lose some of the benefits of a larger system. Operating only during peak electricity cost periods for around half the year can be an effective option. If you are considering trigeneration, then you also need to take into account that reduced electric chiller usage will reduce the electricity demand. There are a few typical dos and don’ts summarised below. Do • consider the heating, cooling and power requirements • implement energy efficiency measures first • allow for cooling tower or dry cooler space increase requirements • start the electricity generation connection negotiation process as soon as possible (as this can take a long time and can be costly) • engage a specialist to undertake a feasibility study including whole-of-life assessments to see if cogeneration stacks up financially • consider all services and architectural requirements. Don’t • look at cogeneration without being clear on the big picture • oversize the unit • forget a maintenance contract • assume you can remove existing plant/grid connections • forget that the plant will need a flue exhaust (typically to the roof) • leave gaps in responsibility (as the plant interfaces with hydraulics, fire, HVAC, electrical, architectural and planning in a larger way than many other projects). Overall, cogeneration can be a fantastic retrofitting option for your building, but it does take some care to get it correct and operating in the way that it is intended to.
Retrofit Volume 1 Issue 2 2012