|Multiple units used to meet a range of demands – three SAV CHP systems|
If CHP is to play its full role in reducing UK carbon emissions, it needs to be ‘empowered’ to deliver its benefits to smaller projects, which often involve distributing hot water around buildings. CHP plant flexibility is key, writes Beata Blachut from SAV Systems.
Very often when people think of CHP, their first thoughts are of large-scale plant designed to meet high but steady power and heat loads. Indeed, many COSPP readers are probably dealing with such large-scale installations on a daily basis.
But to realize its full potential in reducing the UK’s carbon emissions, CHP must be applied to a much wider range of project types – particularly relatively small projects – which bring their own challenges. Not least of these is the inflexibility of traditional CHP designs, which have tended to impose constraints on the environmental benefits that can be achieved.
Many specifiers have also been deterred from using CHP for projects such as multi-dwelling buildings because of the historically poor temperature control within living spaces. Such poor control inevitably leads to complaints from residents and can damage the relationship between the end client and the specifier.
So it is equally important to ensure the efficient use of the heat generated by CHP to give specifiers the confidence to push CHP and district heating into areas where currently they fear to tread. Furthermore, specifiers can add value for the end client by incorporating smart metering to enable accurate monitoring and targeting of energy usage and to facilitate billing for individual spaces where appropriate.
Therefore, the ability to use CHP in more flexible ways, aligning it more closely to the changing demands of the building’s owners, managers and occupiers will be a key requirement for many projects in the future. We would argue that using modulating CHP plants with lower-capacity units in modular configurations, linked to smart heat interface units within the building offers the greatest potential.
For example, conventional fixed-output CHP is usually sized to match the site’s baseload, and makes no contribution to site usage beyond this, which clearly limits potential energy savings.
But electricity generated by CHP units that can modulate their output and track site demand never exceeds requirements, so there is no need to ‘dump’ heat or sell surplus electricity to the grid at unfavourable rates. In this way the CHP units can be matched approximately to the building load and will then ‘self-learn’ and adapt to changing conditions (see Figure 1). In fact, load-tracking, modulating CHP can be thought of as ‘plug and play’.
|Figure 1. Single load tracker CHP – units can be matched to building loads|
This approach maximizes use of on-site power generation and offsets the most expensive utility: electricity. Also, the flexibility of a modulating system means that if the building’s power usage changes – perhaps after energy-saving measures such as a lighting upgrade – the system quickly adapts to the altered loads.
Heating and domestic hot water (DHW) demand patterns in many relatively small applications mean that CHP serving a district heating system can deliver real benefits. Projects that lend themselves particularly well to load-tracking CHP designs include:
- multi-residential housing,
- student accommodation,
- nursing homes,
- leisure centres.
Often, though, CHP is not considered because of the perception mentioned earlier that CHP is only suitable for large projects.
Modular CHP tackles this misconception head-on, because it enables multiple units to be used to meet a range of demands – in much the same way that modular boilers offer greater flexibility over single, large boilers.
Even in larger projects, the need to carefully match a large fixed-output CHP unit to the anticipated electricity and heating loads may impose constraints on CHP’s contribution to the building’s operation. Again, a modular configuration of smaller CHP units will help to maximize CHP usage, thus minimizing the use of grid electricity and other heating plants such as boilers.
With modular CHP, up to five units can be combined to provide a range from 15 kWe/30 kWth to 100 kWe /200 kWth. So one unit may be enough on a small site, while several small units can be combined to meet the needs of larger applications.
This principle was clearly demonstrated recently when SAV Systems evaluated the options for a small leisure centre. With conventional CHP units, sized to cover base electrical demand only, the CHP would only provide 39% of site electricity usage, resulting in energy and carbon savings of about 10%. In contrast, by using a modular configuration of modulating CHP units, 80% of the site’s electrical demand could be met (see Figure 2).
KEEPING FLOW TEMPERATURES CONSTANT
Another challenge for specifiers is that traditional CHP operates with a constant temperature differential (ΔT), resulting in variable flow temperature and inconsistent performance of the system.
The solution to this is to incorporate a heat distributor that maintains a constant flow temperature, corresponding to the design flow temperature, irrespective of the return water temperature. The flow controller in the heat distributor can be set to deliver a heating flow temperature in the range of 40–85°C.
|Figure 2. Three load tracker units can maximize the proportion of site loads met with CHP|
As a result, the CHP always produces high-grade heat that can be used on site without ‘topping up’ from boilers. In fact, as long as the heat loads are within the CHP unit’s capacity, there will be no need to use the boilers.
Furthermore, any surplus heat is stored at 80–85°C and this stored heat helps to optimize CHP operating times and further reduce the likelihood of backup boilers being operated.
INTEGRATING CHP WITHIN HEATING CIRCUITS
Load-tracking CHP can be incorporated into a heating system either in series or in parallel with the boilers. In either case, to achieve the best possible performance from a CHP installation, careful attention must be given to integrating CHP within heating circuits. To address this issue and optimize the system, attention needs to be paid to the whole system design, rather than simply the central plant room design.
In particular, the system should be designed to meet the requirements of CIBSE Guide AM12, which identifies the following key objectives:
- The CHP unit should operate in preference to the boilers at all times.
- When boilers are in service, they should not prevent the CHP unit from operating at maximum output.
- Heat recovery from the CHP unit should be optimized.
- The CHP unit should always be able to generate heat even under part load heating conditions.
- The building heating system should be designed so that return temperatures do not result in the CHP unit shutting down unnecessarily.
MAKING DISTRICT HEATING WORK
Even when the CHP plant is operating at maximum efficiency, problems can occur in the control of space heating and hot water systems served by the district heating system. Indeed, several projects have disappointed their end users and embarrassed the specifiers – often resulting in a reluctance to specify such systems again in the future.
The key to preventing this, and maximizing the potential of CHP, lies in the specification of the heat interface units (HIUs) within each space. This aspect of system design can also have a direct impact on the efficiency of the CHP and other heat sources (see ‘Maximizing Efficiency’ section below).
Standard HIUs often lack useful control functionality, resulting in a lack of responsiveness and poor temperature control of DHW. These issues can be addressed by making use of the right valves within the HIUs.
The heating circuit is designed for direct distribution of heat through the HIU, or indirect distribution of heat through an extra heat exchanger to provide hydraulic separation of the primary and secondary circuits. The differential pressure controller sets the optimum operation conditions for radiator thermostatic valves, enabling individual temperature control in each room. Use of a zone valve with actuator and a room thermostat will enable a time-dependent temperature control programme.
DHW is heated in the heat exchanger and the temperature is regulated with a flow-compensated temperature controller with integrated differential pressure controller. Heat is transferred from the flow water to the DHW via a heat exchanger, ensuring that DHW is delivered at a safe temperature, while the control valve compensates for variable loads, supply temperatures and differential pressures. This protects the heat exchanger against overheating and limescale formation.
Use of fully pre-insulated HIUs will ensure that all of the heat delivered to the space results in useful heat production and that there are no uncontrolled heat emissions during summer months. It also eliminates reliance on on-site lagging and avoids insulation being disturbed during commissioning and not being replaced.
HOT WATER ON DEMAND
Another potential problem is a long delay in the supply of sufficiently hot DHW at times when space heating demands are low. DHW temperatures may also fluctuate in relation to DHW usage in other parts of the building.
|A heat interface unit (HIU)|
This can be avoided by using an integral idle temperature controller in the HIU’s control valve, keeping water in the supply pipe warm so that DHW is highly responsive at all times. The system should also be able to provide thermostatic control of hot water temperature at varying inlet pressures so that DHW temperature is unaffected by opening and closing of other taps on the system.
There are also benefits to including ‘ultra low flow’ balancing modules in the system to ensure temperatures are balanced in each apartment, in addition to reducing flow rates in heating circuits to increase heat exchange and maximize ΔT. This arrangement facilitates compliance with CIBSE Code W and BSRIA Guide BG12/2011 – Design of Energy Efficient Pumping Systems.
Clearly, it is also important that the district heating design facilitates efficient performance of the CHP plant. In this respect, return water temperatures are critical and Part L of the Building Regulations recommends that the return water temperature from a community heating scheme should not exceed 40°C for hot water systems and 50°C for radiator systems.
Low return temperatures increase condensing levels by condensing boilers and also facilitate efficient heat pump performance and maximize the energy transferred to a thermal store from CHP plants. During hot water generation, the design of HIU described above typically returns heating water at 15–30°C, greatly facilitating compliance with Part L.
If the benefits of CHP are to be fully exploited, it is important to think beyond the plant room and address all of the issues that impact on the overall efficiency of the system. In this context, ‘efficiency’ also refers to the delivery of heating and DHW to the users because bad experiences are a real barrier to the wider adoption of CHP/district heating in these applications.
A holistic design that addresses the flexibility of the central plant and the distribution of hot water for space heating and DHW, as well as the influence of the distribution system on central plant efficiency is the only way to break those barriers down.
Beata Blachut is a product manager with SAV Systems, Woking, UK. www.sav-systems.com
Mansfield Halls student accommodation
Mansfield Halls provide accommodation for students at Reading University. To achieve low running costs and carbon emissions the halls use three SAV LoadTracker CHP units with 15 kWe/30 kWth capacity. One of the units serves 147 bedrooms and 14 studios, while a double module serves a further 457 bedrooms.
Combined, these units meet up to 75% of domestic hot water demand and up to 9% of electrical demand, delivering savings in energy costs of over £13,000 (US$21,000) per annum, compared to a mains supply/gas boiler arrangement, with a reduction in carbon emissions of 4%.
The Christie Unit, Salford Royal Hospital
The Christie Unit at the Salford Royal Hospital is one of a network of Christie radiotherapy centres. It has very high energy demands and requires an efficient means of producing heat and power to comply with the Building Regulations.
The solution was to install a single 15 kWe/30 kWth LoadTracker CHP unit capable of supplying 92% of heating and hot water demand and 20% of electrical demand. The result is savings in annual energy costs of nearly £7000, compared to a conventional mains supply/boiler system, with a 10% reduction in carbon footprint.