A flexible friend

CHP and district heating are often perceived as being inflexible and difficult to control, but with the right combination of central plant and HIUs, this needn’t be the case. Beata Blachut and Silas Flytkjaer of SAV Systems explain

There can be no doubt that combined heat and power (CHP) is a tried and tested technology with the potential to significantly reduce energy consumption and carbon emissions. Nor is there any question that many specifiers have certain misconceptions about CHP and district heating. These include inflexibility of the CHP plant and lack of responsiveness and poor temperature control at the district heating heat interface units (HIUs).

The purpose of this article, therefore, is to lay these misconceptions to rest and explain how CHP and district heating can work in harmony to deliver a very responsive and highly efficient solution.

Modulate to accommodate

For example, conventional fixed-output CHP is usually sized to match the site’s base load, so that it does not contribute to site usage beyond base load, which limits the energy savings that can be achieved.

However with CHP units that are able to modulate their output and track site demand, the electricity generated never exceeds demand; thus 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 Fig. 1).

This approach maximises use of onsite power generation and offsets the most expensive utility – electricity. Plus, it provides an adaptable system when, for instance, energy saving measures in the building reduce electrical loads

Modular flexibility

Furthermore, it is widely believed that CHP is unsuitable for smaller projects – even in applications such as multi-residential dwellings where CHP and district heating would deliver real benefits. However, modular CHP 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.

With modular CHP, up to five units can be combined to provide a range from 15kWe/30kWth to 100kWe/200kWth. So on a small site one unit may be sufficient, while several small units can be combined to meet the needs of larger units.

This principle was clearly demonstrated recently when we evaluated the options for a small leisure centre. With conventional CHP, sized to cover base electrical demand only, the CHP would only provide 39% of site electricity usage, resulting in energy and carbon savings of around 10%. In contrast, by using a modular configuration of modulating CHP units, 80% of the site’s electrical demand could be met (see Fig. 2).

Keeping flow temperatures constant

Another challenge for specifiers is that traditional CHP operates with a constant temperature differential, 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 40-85°C.

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’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 optimise CHP operating times and further reduce the likelihood of back-up boilers being operated.

Making district heating work

Even when the CHP plant is operating at maximum efficiency there can be problems with the control of space heating and hot water systems served by the district heating system. Indeed, several projects have resulted in disappointment for the end user and embarrassment for the specifier.

The key to preventing this lies in the specification of the heat interface units (HIUs) within each space. For instance, standard heat interface units often lack useful control functionality, which leads to a number of problems. These include lack of responsiveness and poor temperature control of domestic hot water.

Making use of the right valves within the HIUs will ensure precise control of both space heating and domestic hot water.

In this scenario, 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. In order to enable a time-dependent temperature control program, a zone valve with actuator and a room thermostat can be included as an option.

Domestic hot water (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 lime scale formation.

Another issue to address is the potential waste of energy through standing heat losses in the distribution circuits. The answer is to use fully
pre-insulated HIUs to ensure that all of the heat delivered to the space is used for useful heat production and that there are no uncontrolled heat emissions during summer months.

It also makes installation faster and easier and eliminates reliance on onsite lagging, as well as avoiding the problem of insulation being disturbed during commissioning and not being replaced.

Hot water on demand

With district heating sub-stations there can be long delays 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.

This can be avoided by using an integral idle temperature controller in the HIU’s control valve. This ensures that water in the supply pipe remains warm, so that the DHW is highly responsive – even at times when space heating loads are low. 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.

Maximising efficiency

Usually the reason for specifying CHP and district heating is to maximise energy efficiency, so it is 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. During hot water generation, the design of HIU described above typically returns heating water at 15-30?C, greatly facilitating compliance with Part L.

HIUs can also be fitted with energy meters for monitoring of energy and fair billing of tenants, using either credit based or pre-payment systems.

Given the potential for CHP and district heating to help reduce energy consumption and carbon emissions it’s clearly important to ensure the system delivers maximum performance. Specifying flexible CHP and responsive HIUs is certainly a step in the right direction.

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