Combined heat & power
Finding a fan base in a few new sectors
Ian Hurst, Viessmann’s technical sales manager
widely accepted as the solution of choice for consultants and specifiers for smaller commer- cial 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 sus- tainable and as such it is enjoying market growth – between 2009 and 2012, the market for units reg- istered with CHP Quality Assurance Programme grew on average by 10 per cent per year. Compared with a separate ener- gy supply with power from a con- densing power plant and heat from a boiler, primary energy sav- ings of well over 30 per cent can be achieved. Depending on the fuel comparison, this translates into up to 25 per cent less nitro- gen 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 reduc- tions are nowhere to be seen. Often, a back-up boiler has kicked in, hiding the CHP failure. There are a number of consid- erations when evaluating CHP:
Heat and power requirement While CHP is a good all-rounder, there are applications that suit bet- ter 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 else- where? 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
22 | July 2014 | HVR
Not just for large commercial use, smaller CHP units are attracting new sectors of the market. Ian Hurst, explains some of the key considerations
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 cen- tres, schools and care homes: build- ings which are in constant use. Industrial environments such as factories with machine shops should evaluate CHP very careful- ly. 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
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 temper- ature returns. Systems are con- nected 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 cor- rectly is imperative for them to work effectively, yet many
as increasingly, CHP is being used in conjunction, not only with boil- ers, but with renewable technolo- gies such as heat pumps and solar PV. This is something Viessmann is well placed to support as we manufacture all heating technolo- gies 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 impres- sive CoPs in excess of 10. Viessmann recognises the com- plexities of CHP and, in support of its consultant and installer part- ners, 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 work- ing and provide technical back up.
Viessmann’s Vitobloc 200 CHP unit
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 sus- tainable 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 suffi- cient. 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
approach a CHP in the same way as a boiler.
Another overheating factor, typ- ically 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 sys- tems design approach, particularly
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 pack- ages 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 warran- ty of up to 10 years.
// The author is the technical sales manager at Veissmann //
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