Not just for large commercial use, smaller CHP units are attracting new sectors of the market. Ian Hurst explains some of the key considerations

CHP is widely accepted as the solution of choice for consultants and specifiers for smaller commercial installations where a 20 to 70kW unit will serve both the heating and electricity needs of an average care home, hotel, or leisure centre. The carbon savings targets required by the Climate Change Act of 2008 have made CHP a top box to tick for local authorities and businesses alike.

Though powered by a fossil fuel, CHP is still considered sustainable and as such it is enjoying market growth – between 2009 and 2012, the market for units registered with CHP Quality Assurance Programme grew on average by 10 per cent per year.

Compared with a separate energy supply with power from a condensing power plant and heat from a boiler, primary energy savings of well over 30 per cent can be achieved. Depending on the fuel comparison, this translates into up to 25 per cent less nitrogen oxide and around 60 per cent less carbon dioxide.

While the technology is much better understood nowadays, we are still seeing too many poor installations. Many end-users are unaware there is a problem with their CHP until the electricity bill arrives and the promised reductions are nowhere to be seen. Often, a back-up boiler has kicked in, hiding the CHP failure. There are a number of considerations when evaluating CHP:

Heat and power requirement

While CHP is a good all-rounder, there are applications that suit better than others. Is the CHP to be thermally or electrically led? What are the premises’ heat and power requirements? How does one requirement affect the output of the other and what is the impact on CHP sizing? How much does it cost to acquire that energy elsewhere? What is the spark gap?

As a rule of thumb, a healthy CHP installation is one that will run a minimum of 5,500 to 6,500 hours per year or around 17 hours per day and where there is a constant base load. This makes it ideal for swimming pools and leisure centres, schools and care homes: buildings which are in constant use.

Industrial environments such as factories with machine shops should evaluate CHP very carefully. Here, CHP may be attractive for its electricity generation but it is hard to utilise the heat load. Add the dimension of hot machinery in summer and the demand will be for cooling, not heating.

Excess heat can be blown into the air and dumped, which has been a common procedure for larger scale CHP where the systems have been designed for it but it is apparent this is not the most sustainable solution. Climate change levies are due to put limits on the extent to which that can be done.

Sizing

A lot of CHP units are mis-sold with regard to oversizing. They perform best when running at full capacity and surprisingly a smaller unit is often a more suitable choice. Yet so many installers and consultants working with CHP for the first time will specify a 200kW unit where a 20kW would be sufficient. The trend is to treat CHP like a boiler, believing boiler output should be replaced like for like.

A 39kW thermal output is more than adequate for a 45, two bed, apartment building when using buffer vessels to cover hot water peak load.

High temperature returns

The longest standing issue behind failing CHP units, and one which unfortunately gave the technology quite a bad reputation around five years ago, but was entirely due to installation set up, is high temperature returns. Systems are connected in such a way that water of too high a temperature is fed back into the CHP causing it to fail to operate. Piping CHP systems correctly is imperative for them to work effectively, yet many approach a CHP in the same way as a boiler.

Another overheating factor, typically in nursing homes, is where maintenance staff, constantly turn up the heating, even in summer. We suggest that our customers lock down heating and discourage fiddling which can inadvertently result in high temperature returns.

It’s shocking to visit premises where the CHP has not run for 13 years just from poor piping and where simply stabilising the return temperature down from 71 to 65°C would have solved the problem.

System design and installation

The previous issues highlight the problems arising from treating CHP as a bolt on. We believe all heating systems and their controls should be part of a holistic systems design approach, particularly as increasingly, CHP is being used in conjunction, not only with boilers, but with renewable technologies such as heat pumps and solar PV. This is something Viessmann is well placed to support as we manufacture all heating technologies with a single control platform, designed to communicate and integrate effectively.

CHP can work very well with heat pumps and underfloor heating where heat pumps can be used to raise the low temperature return. In turn, the CHP-generated electricity powers the heat pumps, resulting in a system that can return impressive CoPs in excess of 10.

Viessmann recognises the complexities of CHP and, in support of its consultant and installer partners, collectively approves every design prior to installation and commissioning. Back at base, thanks to the technology systems and controls in place, we can monitor how the system is working and provide technical back up.

Maintenance and servicing

The best way to approach CHP maintenance and servicing is to think of it like a car. After all, the heart of a 20kW system is a two litre engine.

CHP modules amortise within a few years, especially if they achieve long running operating times and the generated power is consumed. To achieve the planned amortisation, CHP units must be serviced regularly.

To make this as hassle-free as possible and to completely control operating costs, Viessmann has launched a range of service packages for its Vitobloc CHP units. At three different levels, customers can opt for a standard service schedule package or increase to a contract that includes unscheduled visits and replacement parts; equivalent to an extended warranty of up to 10 years.

// The author is the technical sales manager at Viessmann /