Before purchasing a cogeneration system, the steps to be taken are different and fundamental for the achievement of the objectives that these machines aim to solve: economic savings, continuity of service and lower environmental impact.
In the cogeneration proposal phase, the energy profile of the users first takes on importance, after the choice of technology and finally the sizing of the plant. A CHP system implies a complex plant and the economic convenience is strongly dependent on the energy profile of the final user. The privileged users for cogeneration are those characterized by a rather constant demand over time for thermal and electrical energy in order to make the most of the machines that are designed to work 24/7 365 days a year (except for periods of stops planned for maintenance). Usually, they obtain extremely good results when working 7000-8000 hours a year. Enough good results may already be achieved when working hours are above 4000 h per year.
Producing simultaneously electricity and heat means that in the case of lower demands of one of the two carriers there is an excess of availability of the other.
Excess of electricity:
1) sale to the grid at a price of kWh lower than the cost of production;
2) accumulation in the grid to be recovered at the appropriate time, a charge for the service shall be recognized at the time of the adjustment.
The way in which a cogenerator is connected in an electrical system comes technically with the following terms:
Island operation: when the cogenerator is not connected to a public network, i.e. it supplies the users separately.
Parallel operation with the public grid: when the cogenerator is connected to a public grid both for functional reasons and to integrate its own production, i.e. it is connected in parallel with the public grid.
Mixed operation: when the cogenerator normally operates in parallel with the public network or on an island. In this way, the cogenerator is generally used for the emergency service in periods when the public network out of order for some reason.
The connection is always subject to technical feasibility and is carried out either by the person who provides the sales service of the system or by the distributor, which must be stipulated as an operating regulation.
Depending on the type of plant where the cogeneration unit is installed and on the needs of the customer, different operating systems are possible, each of which can be implemented with a different of the automatisms of the electrical control panel, which also integrate the necessary protection systems of the cogenerator.
Excess of thermal energy:
1) dissipated into the atmosphere. If this mode is prolonged for significant periods, it would confirm the oversizing of the thermal section of the cogenerator (greater investment with direct influence on the economic budget);
2) thermal storage. Increases the flexibility of the system but involves an economic burden. Most cogeneration plants do not have large thermal storage capacity.
The best way not to dissipate any type of thermal energy is to connect to the traditional system of boilers that are already installed in the building, so as to activate them only when the demand cannot be supported by the use of the cogeneration system. In the periods when high demand peak is not reached, compared to the average, the boilers will be inactive and the user will be able to enjoy the heat recovered free of charge from the CHP system. The logic expressed up to now is functional when a careful analysis of sizing is carried out by the company providing this service: it is precisely for this reason that this type of technology requires careful attention to the preliminary feasibility study and is not suitable for standard solutions that are already predefined.
After the feasibility study, a visit will have to be made by specialized technicians to decide where to position the cogenerator: there are both indoor and outdoor solutions that give this phase certain flexibility. In any case, the CHP is normally installed not far from the user and connected to the existing electrical panels and thermal piping of each facility in a simple way, to provide the energy necessary for the operation of the structure.