Alex Parkinson of Bosch Commerial Heating evaluates the pros and cons of multiple module CHP systems vs single module arrangements

With ChP still generally viewed as something of an emerging technology, discussions around the best design practices are still ongoing. As with any appliance within the heating and hot water industry, there is no ‘one solution fits all’ approach, which means consultants and contractors always need to make a multitude of performance-related considerations. As far as this is concerned, there appears to be differences of opinion across the industry with regard to when cascades of multiple CHP modules, and single module systems should be used. Generally speaking, the conclusions lie in the assessment of four key areas.

Installation cost

When it comes to the time and cost involved in the installation of a CHP system, a suitably-sized single module inevitably boasts the fewest complications and subsequently, the most cost-effective installation. The delivery and installation of one module means that only one set of gas, electrical, BMS, and flow/return connections needs to be made, and only one meter required. As a result, the installation and commissioning procedure ought to be relatively straightforward.

By contrast, multiple units require an increased number of ancillaries. A system comprising three CHP modules for example, would require three of each connection to be made, three meters to be fitted, and three commissioning procedures to take place. This extended list of requirements results in longer installation time, and added labour costs.

Plant space

As with any commercial or industrial heating and hot water technology, an investment in CHP will have spatial implications and as a result, careful planning of the plant room layout will be required. Naturally, one of the major drawbacks of a system comprising multiple CHP modules is that more plant space is generally required. The physical dimensions of each CHP module isn’t the only logistical consideration to be made here either, as each module will generally require a connecting buffer vessel and a certain amount of clearance to be able to operate to its design potential.

Load mapping

The most effective way to design a CHP system is to align the electrical output as closely as possible with the load of the application. Whilst the advantage of a CHP cascade is the ability to track electrical load to a lower output in the event of a reduction in demand, this can also prove to jeopardise efficiency levels. Ultimately, the full benefit is only realised when the number of hours that the system runs at full load is maximised, so having modules within a cascade tracking electrical load to lower outputs shouldn’t necessarily be seen as a desirable option.

Furthermore, instances in which a load as little as 10kWe is required tend to be few and far between. In the event there is no demand for heat and therefore very little financial saving potential, the maintenance costs of a multi-module cascade would make it not economic to operate.

As an alternative, a single module system can, if the manufacturer’s product specification allows, modulate both electrically and thermally to 50 per cent of the load. While this level may not be as large a reduction as with a three module cascade for example, having a system designed to modulate for long periods is not a strong design scenario as the cost generally far outweighs the benefit.

Standby

For investors, one of the biggest attractions of a system comprising multiple modules is that should one unit fail, the facility in question will still be able to rely on the remaining units to deliver a supply of heat and power. That said, the reduced collective output of a system where one module is down limits the financial gain of the system, as the capital cost of two or more units plus two or more aintenance contracts would result in a significantly reduced payback.

When it comes to the effective observation and maintenance of a CHP system, manufacturers such as Bosch offer a remote CHP monitoring system to respond immediately if a faulty connection occurs. With CHP downtime having the potential to damage financial and carbon savings, remote monitoring can reduce this risk, as well as offering stakeholders the peace of mind that can prove so important when it comes to large scale investment.

Although it is usually possible to meet the requirements of an end-user with the design and installation of a single or multiple CHP system, it is important to consider the advantages and limitations of each approach beforehand. Recent industry trends sug gest that there is a tendency for consultants and contractors to favour a cascade of multiple units; however a single CHP module can often be a more attractive proposition – especially when it comes to installation and maintenance costs. As the range of CHP outputs offered by manufacturers continues to grow, stakeholders will be able to benefit from greater versatility and assurance that a single module can be sized according to the thermal base load of a project.

Ultimately, the aim for the investor is to have a system in place which maximises efficiency levels and offers a favourable payback period.

// The author is the commercial sales manager for CHP at Bosch Commercial Heating //