When in the midst of the exam season, teachers were always fond of saying the following: 'fail to prepare, prepare to fail'.
Fast-forward to working life and the same truth applies, particularly to specifying combined heat and power (CHP). Here, if you fail to plan and do your homework properly you're likely to get a 'U' for 'unfit' grade CHP system, rather than being regarded as an 'A' level performer.
The effectiveness of a CHP system depends in large part on the preparation time and effort spent on ensuring accurate specification.
Your CHP specification is an essential document that describes the physical and operational characteristics of the unit and control systems and how they affect other parts of the building. Determining the optimum capacity for the CHP unit to provide the best returns – whether they be financial and/or environmental – is key to this.
To ease the burden of preparation, here are five things you should focus on to achieve the best specification possible for your CHP:
Identify the goal
CHP can reduce the running costs of a building, reduce its CO₂ emission levels and generate electricity for use on-site, or sale to the grid. It is necessary to understand how and when the energy will be consumed and to identify priorities during the design process, such as whether there is a primary goal and if it is related to sustainability, cost reduction or both.
For example, where generation of electricity is the priority, it may be acceptable to size and design the system to reject small amounts of excess heat at times when electricity demand is higher than the associated heat demand.
However, if the priority is to reduce CO₂ emissions then a strategy that rejects heat will not be appropriate, as this will dissipate any emissions reductions obtained through CHP. It is key to strike the right balance.
Work out the demand
In order for a CHP plant to be financially viable and achieve the desired cost savings and payback period, it must run at full capacity for at least 4,000 to 5,000 hours per year to achieve maximum efficiency.
The unit must, therefore, be sized from the heat and electrical power profiles of the site, which should be studied on meter readings on a daily and seasonal basis. Depending on the site, the CHP system should be matched to the thermal (heat) profiling of the site.
A system sized against the thermal baseload of the site is likely to be able to run efficiently all year round, however the savings may not appear to be as great as initially expected.
Conversely, a system sized against the electrical baseload of the site will be able to deliver more savings in the winter months, but will have to reject heat or even shut down in the summer months.
There is an optimum size CHP which sits somewhere between these two limit cases, but it is only by having a comprehensive understanding of the site's energy profile that a skilled CHP engineer can determine the best solution.
To reject, or not to reject
Where the priority is electricity generation, it may be desirable to run the CHP plant to produce electricity even when there is little or no demand for heat, with the unwanted heat being rejected through an air blast radiator or similar device.
This will be economically viable if the grid electricity price is high enough, but it will have a negative impact on CO₂ emissions. Too high a proportion of heat rejection will negate much of the benefit of CHP. It is also likely to fail the government’s quality standard for CHP, which only allows 25 per cent of heat to be rejected.
Use realistic comparators
The investment case for CHP is inherently comparative, as its viability depends on the difference between the cost of providing heat and power through CHP and the cost of conventional heat sources and grid electricity.
CHP performance must be evaluated against a realistic comparative case, which uses agreed baseline energy costs for gas and electricity. Markets have seen continuous increases in the price of electricity – and with gas prices staying relatively static in comparison – are prime to benefit from CHP technology.
Future-proof your project
A CHP plant is an investment with a lifespan of 15 to 20 years, although the payback period is usually three to five years for well-specified systems. It is, therefore, essential that the financial case remains viable for at least the payback period.
The basic measure which determines the financial return on CHP is the spark spread. That is, the difference between the market cost of electricity and the cost of production in the CHP plant. Although it is not possible to predict future spark spread with complete accuracy, a realistic business case for CHP must take the effect of long-term price variations into account.
Read more about drafting a quality specification
here.
Author: John Hyde is a Cork-born mechanical and manufacturing engineer with more than 15 years’ experience in low-carbon technology in the UK, US and Europe; focused on a 'triple bottom line' approach to an economic, sustainable and social benefit through technologies, in particular Cogeneration.