Author: Fergus Cawley, senior electrical projects engineer, Biop
Combined heat and power (CHP), also known as cogeneration, involves the simultaneous production of heat and electrical power from the same source of fuel, usually natural gas. The CHP consists of a gas-fuelled reciprocating engine, a generator/alternator, and a heat recovery system capturing exhaust gas and engine heat.
A CHP uses the by-product heat that is normally discharged to the atmosphere resulting in a higher thermal efficiency. This leads to reduced energy costs and, as a result, CHP engines have a lower rate of carbon dioxide (CO2) emissions per megawatt (MW) of useful energy produced. The heat harnessed from the machine is available as low-pressure hot water and steam.
A CHP unit also offers greatest benefits where the heat load is large and constant throughout the year and makes excellent economic sense when the ratio of grid electricity prices to gas prices per kWh are high. A brief explanation of all key elements to consider are detailed bellow:
This is the ratio between the cost of a unit of electricity versus a unit of gas. A ‘unit’ is a kilowatt-hour, which is one kilowatt of power for 1 hour. So, this measure of energy is the same whether for gas or electricity. For CHP to be viable, the cost of a unit of electricity, divided by the cost of a unit of gas, generally need to be 2.5 or greater. The higher this gap, the greater the benefit of the CHP plant and the shorter the payback period.
- Thermal (Heat Load) Profile
A CHP unit needs to be able to effectively dissipate the heat generated as a by-product of combustion. This heat is harnessed from the various engine circuits (such as engine coolant, lube oil coolers and turbocharger intercoolers) by a series of heat exchangers and is available in the form of hot water and steam and can be used directly in the plant rather than using hot water and steam boilers.
When sizing a CHP unit, the thermal load must be monitored to ensure that the heat-load is present to ‘sink’ the CHP heat, and therefore benefit the customer. Uses of steam and hot water can vary from humidity control, space heating, jacket heating, washing and cleaning skids and sterilisation.
- Electrical and Thermal Efficiencies
The electrical efficiency is extremely important in CHP selection, as it tells you what percentage of the input fuel energy goes to generating electrical power. As electrical power is expensive to buy from the grid, the cheaper a CHP can generate a kWh of electricity, the bigger the payback for the customer. Modern CHP units can have electrical efficiencies of up to 45.7 per cent. The thermal efficiency is a measure of the usable heat generated per kWh of input fuel.
- Availability (Uptime) Guarantees
A CHP vendor will offer an availability guarantee, which will guarantee the customer that the machine, when properly maintained as per manufacturer’s guidelines, will run for a minimum number of hours in the year. This figure is usually expressed as a percentage. So given that there are 8,760 hours in a year, a typical up-time guarantee would be given as 92 per cent, which equates to 8,059 hours.
Penalty clauses should be in place so that the vendor is penalised if the guarantee is not met. The availability guarantee is a measure of the vendor’s confidence in their equipment and design, as well as in their ability to maintain the equipment satisfactorily. A vendor will insist upon being able to remotely monitor the machine as it operates, so that any alarms can be highlighted early and dealt with to minimise down-time. This remote monitoring is usually configured using a broadband-enabled telephone line (internet connection).
- Operations and Maintenance Costs
Modern CHP units are highly efficient machines that need regular scheduled maintenance in order to guarantee their reliability over their lifetime. The cost of maintaining a multi-cylinder CHP machine can be expensive and needs to be factored into the overall payback calculation figure to determine viability. The vendor will usually quote for this cost in a €/hour run format. It can also be quoted as a cost per electrical kilowatt hour (kWhe) produced.
As well as routine maintenance, these machines will require scheduled minor, intermediate and major overhauls. Maintenance quotations can be structured to include (or exclude) these larger services and overhauls depending on available finance.
- Installation and Integration Costs
Installing and integrating a large scale CHP into an existing plant can be expensive and invasive. Such an installation needs to be professionally assessed and designed to ensure optimum design and operating efficiencies. There are tie-ins required to a gas supply, a low-pressure hot-water loop, a steam loop, the site electrical ring main system and plant building-management system. These design and installation costs need to be included in the overall payback calculation to determine CHP viability.
Biopharma Engineering has extensive CHP experience with design, installation and commissioning of CHP systems and combined cycle power plants in pharmaceutical, medical devices and educational facilities. It can assess CHP viability and payback times, initial requirements and project scope development, as well as project manage and deliver turnkey CHP solutions.