CHP Designs

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This article is dedicated to systems making use of a CHP (Combined Heat and Power) engine.

The starting point will be a Spice2e Gas Fired CHP unit, with 2 kWe and 7.5 kWt.

The system needs to be run flat out, and as electricity can be sold to the grid, the system becomes heat led, and requires a 7.5 kW base heat load for ideal operation. This is equivalent to 180 kWh.

As the majority of the year there is no central heating requirement, the system needs to be matched with hot water demand.

Figures for hot water usage vary from 45 to 65 litres per person per day under normal circumstances having a five minute shower each day. In reality, it can be lower due to day-to-day occupancy levels and simple prudence. It can also, with certain shower fittings be far higher. A 30 litre per minute shower rose for 10 minutes is 300 litres, so its always checking customer expectancy levels. A bath takes between 65 and 150 litres.

Working for now with a figure of 65 litres, equivalent to 3 kWh.

Each CHP unit therefore is matched to a basic hot water demand for 60 people.

Each of the 60 people will have 33 Watts of electrical power, equivalent to an LCD TV or some light bulbs.

A feel for the timing of electricity use can be seen from page 193 of Household Electricity Survey

Equipment Selection

We have a CHP heat source that can deliver high grade heat over 70C, and a purely hot water load for roughly 60 people.

Heat input is a steady 7.5 kW. Equivalent to 3000 litres of stored hot water per day.

2000 litre Heat Bank

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Hot water load is spread out with three peaks - morning, lunch, and evening.

A suggested storage size of 2000 litres would be made to allow for concentration of load.

Given we would like to absorb occasional heat if the load shifts around, and given we are looking at central storage (first case), then a thermal store using a heat exchanger system would be our usual offering.

If there are unvented cylinders in place already serving load, then a plate heat exchanger recovery system would be added, driven from the thermal store. It would be important to interlock any existing heat sources with the CHP to be held off unless ever needed as backup.

Also given that contracts are typically retro-fit, adding a CHP unit into an existing plant, the use of a thermal store lets us drop store pressures and make use of rectangular 2000 litre stores we have used in great effect in the past. They can be fit through many a front door or into gaps where regular 2000 litres of storage just will not fit.

The store is to be supplied with the following controls factory fitted on one end, wired and tested:

  • Return temperature control on CHP recovery circuit
  • Plate heat exchanger set for DHW
  • Temperature sensors
  • Minibems Controller
  • Wiring centre
  • Heat Meter

Where there is a fresh installation, it is recommended to use the thermal storage system outlined above in partnership with an unvented cylinder of 500 litres, with large inlet and outlet connections. This allows it to satisfy spikes in demand, while the plate heat exchanger recovery system allows heat from the thermal store to be rapidly dumped into the unvented system to keep at with peak loading. The 500 litre store would also have heat exchanger inputs for any other heat sources, such as gas boilers.

Unvented

Where DHW load is spread out over the day rather than potentially concentrated in morning and lunch draw-offs.

The most economical means of providing 600 to 1000 litres of storage is using two standard indirect unvented cylinders of 300 or 500 capacity, used as a pre-heat to an existing system, or via a transfer pump.

Pre-heat systems can become troublesome where it requires breaking into large bore existing supply pipes, or where existing stores have a very high pressure rating. In-line pipe size should be maintained. This is where a transfer pump becomes more attractive as connection and pipe size is sized to transfer rates rather than line size.

Care should always be taken with DHW storage systems to ensure they achieve full temperature on a regular basis for Legionella protection.


See also Amazon HXIN Unvented DHW Cylinder

PHEs with No Additional Storage

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This option becomes possible where there is already 2000 litres of storage in place. As long as the storage is heated currently by an external heat source (i.e. not an integral burner), then the method of reheating it can be altered to using plate heat exchangers. The CHP is connected also using a plate. This allows us to manage the recovery from all heat sources efficiently, and accurately control the volumes heated by the boilers to leave the rest or the CHP.

The use of a top-down plate recovery system allows small volumes of hot water to be prepared instantly (like a combi-boiler) without heating up additional storage. Coils on the other hand occupy a fixed position in the bottom half of a store, and are an all or nothing approach that destroys the 'cold water reserve' needed for the CHP to work on.

With two or more PHEs (one for CHP and one or more for boilers) required, as well as controller, The Cupboard HIU would be a typical offering, providing all the above in a space saving, insulated, and neat package.

HIUs

The the proposed thermal store system, it is also possible to generate hot water at point of use, using Heat Interface Units. There are three models to select from:

  • The SLIM HIU generates hot water alone
  • The DIGI HIU generates hot water and manages direct central heating
  • The DATA HIU generates hot water and provides an isolated central heating system

An HIU system will overcome the need for distribution pipework relating to domestic hot water - delivery, and secondary return. It also overcomes all Legionella fears without an stored domestic hot water.

Control System

The control system selected would be the Minibems. This is a multi-purpose controller that can be setup to accommodate variations in installations. It provides pump speed control to enable the most efficient heat exchange policies to be employed.

It will also track data from heat meter, as well as electricity meters, and will be able to provide both the managing agents and the end user with customised web interfaces to view operation and efficiency.