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Hospital HVAC - Cogeneration
Hospital HVAC - Cogeneration

Cogeneration is combined generation of heat and electricity. The advantage of cogeneration is that it enables the heat released by electricity generation to be used. The most well-known form of cogeneration is an electricity generator powered by an internal combustion engine, with the heat released by the engine being used to produce steam and/or hot water. An important factor for the economic potential is that good use has to be made not only of the electricity but also of the heat. This means that there has to be a continuous demand for heat as well as electricity.

Because of their specific heat and electricity consumption, hospitals are particularly suitable for cogeneration. A characteristic of hospitals is that there is a constant demand for heat all year round, for e.g. hot tap water and sterilisation. This constant heat demand profile has a favourable effect on the number of hours at which the cogeneration unit can operate at full load.

Examples of the heat and electricity demand profiles of a hospital are shown in the figures below.

Figure 3 (left). Annual profile for heat demand, Figure 4 (right). Annual profile for electricity demand
Figure 3 (left). Annual profile for heat demand, Figure 4 (right). Annual profile for electricity demand

In practice a cogeneration unit is frequently combined with an absorption cooling machine in order to raise the number of hours at full load in the summer. Note however that it is important to make use of the absorption cooling machine in the correct way, as shown in the calculation examples below.

A further advantage of a cogeneration unit is that it acts as a backup power supply, thus contributing at least part of the necessary continuity of operation in case of a grid power failure; a cogeneration unit can take over part of the work of an emergency generator. When installing a cogeneration unit there a number of important factors that have to be taken into consideration. These are illustrated in the following calculation examples.

Calculation example for a cogeneration installation

In this calculation example a number of practical considerations are examined in more detail. We first look at the costs of the energy generated by the cogen unit.

Electrical efficiency of cogen unit :35 %
Thermal efficiency of cogen unit :50 %
Gas price :30 €/MWh
Price per MWh of electricity (from cogen, without using the heat) :85 €/MWh
Price per MWh of thermal energy (from cogen, without using the electricity) :60 €/MWh
Cost of buying electricity from outside (peak) :80 €/MWh
Cost of buying electricity from outside (off-peak) :45 €/MWh
Cost of thermal energy from gas-fired boiler (eff=90%) :33 €/MWh

From the above data there is clearly no point in depending on the cogen unit only for electricity or only for thermal energy. A number of calculation examples are shown below, based on the above data.

Cogeneration operating during the day and during the night

The differences between peak and off-peak prices for electricity have a strong influence on the cost savings that can be achieved with cogeneration. For the sake of example we will assume that 100 MWh of electricity is produced during the day and that good use can be made of the heat.

During the day:

Electricity produced :100 MWhe
Heat produced :143 MWhth
Cost of electricity + heat produced :8 570 €

In this case 100 MWhe and 150MWhth cost € 8 570. If this quantity of energy had been purchased from outside during the day the cost would have been as follows:

Purchase price for 100 MWhe :8 000 €
Purchase price for 143 MWhth (gas) :4 760 €
Total purchase price :12 760 €
Cost saving due to cogen :4 190 €

If the same quantity had been purchased from outside during the night the calculation is as follows:

Purchase price for 100 MWhe :4 5 00 €
Purchase price for 143 MWhth (gas) :5  000 €
Total purchase price :9 500 €
Cost saving due to cogen :930 €

From this we can conclude that the cost savings achieved by cogeneration depends to a large extent on the energy prices and whether good use can be made of the heat produced. The electricity demand for a hospital is so high that in most cases there is never any surplus. But even if there is a surplus it can be sold back to the grid. For this reason it is important for the cogeneration to be operated on the basis of the demand for heat.

Cogeneration in combination with an absorption cooling machine

In the example below the cogen unit is operated in combination with an absorption cooling machine. In addition to the absorption machine a number of compression cooling machines are also available. The demand for cold amounts to 500 MWhth. The absorption cooling machine has an efficiency of 70%.

The absorption cooling machine is operated whenever the heat demand of the hospital is lower than the heat supplied by the cogen unit.

Amount of demand for cold :500 MWhth
Efficiency of absorption cooling machine :70 %
Heat demand for absorption cooling machine :715 MWhth
Amount of electricity produced (for 715 MWhth) :500 MWhe
Total cost for 500 MWh of cold and 500 MWh of electricity :42 900 €

If the cold required during the day was produced by the compression cooling machines, the costs would be as follows, with the 500 MWh of electricity being bought in from outside:

COP compression cooling :3,5
Electricity consumption by compression cooling machine to produce 500 MWhth of cold:143 MWhe
Cost of 500 MWhth of cold :11 430 €
Cost of 500 MWhe of electricity :40 000 €
Total cost for 500 MWhth of cold + 500 MWhe of electricity :51 430 €
Cost saving :8 530 €

From this we conclude that the absorption cooling machine can usefully supplement the cogeneration during the day, if the demand for heat is low. If we look at the same situation during the night the calculation is as follows:

COP compression cooling :3,5
Electricity consumption by compression cooling machine to produce 500 MWhth of cold:143 MWhe
Cost of 500 MWhth of cold :6 435 €
Cost of 500 MWhe of electricity :22 500 €
Total cost for 500 MWhth of cold + 500 MWhe of electricity :28 935 €
Cost saving :-13 950 €

From this we conclude that it does not make sense for the cogen to produce extra heat at night for the absorption cooling machine. Because of the low price for off-peak electricity, it is better for the cold to be produced by the compression cooling machines.

During the night the cogen would only produce the heat that can usefully be employed, with the cogen being controlled according to the heat demand from the hospital.

SOURCE: LEONARDO ENERGY, Rob van Heur

About Author //

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Edvard Csanyi

Edvard - Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV high power busbar trunking (<6300A) in power substations, buildings and industry fascilities. Designing of LV/MV switchgears.Professional in AutoCAD programming and web-design.Present on

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  1. […] options.These are subdivided into the following areas:- HVAC – Compressed air – Steam – Lighting – Cogeneration For each of these areas we first give a general explanation, followed by some practical examples […]

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