By utilising industrial automation technologies, Rockwell Automation shows how it is possible to manage a modern district heating plant based on geothermal energy
Protection of the environment and energy savings remain high on the public agenda, due to both the rising cost of oil and gas, and the need to limit emissions of pollutants into the atmosphere. Despite a growing awareness, specific applications continue to be limited to the extent that, in Italy, less than seven per cent of energy used is produced by renewable resources.
However, the Santa Fiora district heating plant in Grosseto province uses geothermal energy from the slopes of Monte Amiata to produce heat and electrical energy. The Enel Green Power plant uses water vapour, which uses turbines to produce electricity. When the water drops to a temperature too low to be able to operate the turbines, it is used for heating: in this way it is possible to supply heating remotely to offices and homes over an area of 480,000sq.m, making a total of around 163,000MW/h each year.
Using water vapour present in the subsoil makes it possible to produce heat and energy at virtually no cost and, more importantly, without emitting any pollutants. Despite this, however, the focus on cost efficiencies is very strong for Enel Green Power. During the planning phase, attempts were made to contain realisation and maintenance costs, while providing service continuity. Any failure of the plant would leave thousands of people without heating. For this reason the local district authorities set stringent service levels for the companies responsible for building the district heating plant.
Infrastructure management
Aster of Vimodrone and S.U.D. Impianti of Tivoli, both specialists in this area, opted to delegate the task of automating the plant to PSC Engineering & Contracting, a company with vast experience of water treatment. This decision was based on the experience gained by the company in the automated management of major infrastructures, such as the Milano sud purifying plant.
Under the terms of the contract, it was first and foremost necessary to identify reliable, cost-effective components. The choice was made to use the solutions proposed by Rockwell Automation. In particular the Allen-Bradley SLC-500 PLC, that has been fully tested by Rockwell in hostile environments and offered at a particularly competitive cost. What is more, the SLC-500 PLC is easy to maintain, further reducing costs.
It offers 300 I/Os, which do not require a real time acquisition speed, since any delay of a few seconds in the opening of a valve or activation of a pump has no particular repercussions. However, to avoid risk, reliability and redundancy do have to be provided — and are offered by the SLC family. Rockwell also supports the production and availability of parts for several years to come, soy avoiding the risk of having to carry out major work should the plant be upgraded or repaired.
The architecture constructed is completely original, while the redundancy is provided directly by the software. In fact, the latter operates with two sets of controllers, with the capacity to enter into operation alternately if need be.
Consistently hot water
In a district heating plant which uses geothermal energy, using waste vapour from the nearby power plant, the variables are numerous. Nonetheless, the supply system must maintain a constant pressure (set at 1450kPa) and the temperature of the output water (67°C) must also be consistent. For this reason, three fixed-speed pumps and one variable-speed pump have been set up in the area of the exchangers. The automatic control makes it possible to operate directly on the heat exchange level, adjusting the speed of the rotors to provide the conditions required to appropriately heat the homes connected to the system.
This can only be achieved by using the right automatic system with the capacity to keep the difference between the delivery pressure and the return pressure constant, and so prevent any problem with the distribution system.
Thanks to the work of the PSC engineers, the whole system, in normal conditions, is able to operate completely independently. Nonetheless, if any part of the thermal distribution and exchange system fails, the engineers must take prompt action. For this reason, through the redundancy of the automatic control, the management system has the capacity to help overcome any hardware or software fault.
But the same redundancy cannot be applied to the pump/pipe assembly used for distribution. For this reason, in the event of failure, the intervention of special engineers must be sought immediately. The Rockwell Automation RSView 32 platform supports this task efficiently by using remote control. The same RSView system is also designed to send messages via a GSM modem, so it is possible to report the fault immediately to the duty engineer, highlighting the cause of the problem and the urgency of the action to be taken.
Meeting peak demand
Lastly, the automatic control has also been extended to the water storage system. This tank has been designed to ensure that the system has no trouble meeting requirements at peak demand from the 112,000 families connected to the system. So the management system also automatically controls the temperature of this volume of stored water, which is available for use, if necessary. The same technology has the task of managing the expansion vessel and water softening system necessary to prevent the build-up of limescale inside the pipes.
All the important information relating to operation of the system is recorded in a log-file, containing a log of events which is very useful in the event of failures. Since this is a relatively small system, the information is presented in the form of a table, also because, in most cases, it is enough to review trends in order to establish the need for intervention in a proactive way.
On the screen of the control system, however, a particularly intuitive interface system has been devised by PSC engineers. In this way, staff training is kept to a minimum, and routine management can be effectively performed even without the need for specialist engineers.
