Turin – towards a smart energy city
The north Italian city of Turin already has an extensive district heating system (DHS) supplied by CHP plants and boilers. Now, under an action plan for sustainable energy, the DHS will be extended with additional CHP plants and – crucially – large heat storage facilities, write Gian Vincenzo Fracastoro and Alberto Poggio.
|Figure 1. Turin Action Plan for Sustainable Energy – main actions planned|
Italian and European cities are increasingly adopting combined heat and power (CHP) plants linked with district heating networks, while public interest and research in the field is also rising.
Many studies into urban pollution and primary energy use have demonstrated district heating systems’ benefits for energy efficiency and the environment.
Research has focused on carbon dioxide and pollutants such as particulate matter and nitrogen oxides. New parameters have been developed to measure the drop in pollutants through combined production of heat and power. New scenarios such as low-energy residential buildings have also been investigated.
As a test case, this article analyzes Turin, the largest district heated city in Italy with more than 400,000 connected users. It examines the key plans for changing the network into a smart thermal energy grid, with multiple sources connected to it and an increasingly efficient building stock.
Finally, it examines the potential gains for grid management and primary energy use from using large heat stores located at strategic positions within the network.
THE ENVIRONMENTAL CHALLENGE
The City of Turin is in the western part of the Po valley, surrounded by the Alps on three sides and hills to the southeast. Torino was the first Italian capital (1861–65), and is today the fourth largest Italian city after Rome, Milan, and Naples, with 910,000 inhabitants on a mainly flat area of about 130 km2.
The city’s population fell from a peak of 980,000 in 1991 down to 897,000 in 2002, and is now 911,000.
Turin has long ranked among Italy’s leading industrial cities. But a steady reduction in Fiat’s car plants has slowly shifted the city’s economic structure towards services, higher education, sport and tourism. Turin hosted the Winter Olympic Games in 2006, and is now Italy’s sixth most popular city among tourists.
Meanwhile, Turin has greatly improved its environment, with 20 m2 of green space in the form of parks and historic gardens for each ot its inhabitants. Air quality has improved with the closing of coal- and heavy oil-fired plants.
But, although the last 35 years have seen concentrations of sulphur oxides fall by two orders of magnitude, the situation remains poor in terms of nitrogen oxides, ozone and particulate matter. This is mostly due to a combination of heavy emissions, mainly from traffic, and local meteorology, which is typified by frequent winter thermal inversions and very low ventilation year-round.
A recent survey by the Italian Statistical Institute (ISTAT) of European cities for 2004–08 using data from the European Environment Agency’s (EEA) AirBase found that Torino ranks 220th in a group of 221 European cities for air quality. The analysis was based on a single indicator – the average number of times that legally defined concentration limits were exceeded for the three pollutants mentioned above: nitrogen oxides, ozone and particulates. Legal concentrations were exceeded, on average, by a factor of 2.5.
ACTION PLAN FOR SUSTAINABLE ENERGY
Turin has decided to elaborate the Turin Action Plan for Sustainable Energy – TAPE – to reduce its carbon dioxide emissions by 2020. TAPE reports the results of the baseline emission inventories for the years 1991 and 2005, and the carbon dioxide emission estimates for the year 2020. Carbon dioxide emissions have already been reduced from 6.3 million tonnes (Mtonnes) in 1991 to 5.1 Mtonnes in 2005 – a 19% reduction including the industrial sector.
TAPE envisages a further reduction in carbon dioxide emissions of 1.36 Mtonnes from 2005 up to 2020. Costs are estimated at about €2 billion ($2.4 billion), and much higher if investments by public and private partnerships are included.
The main elements of the project are shown in Figure 1. These include changes to the municipality building stock, to the residential and service sectors, and to transportation. Industry has not been included in the analysis.
DEVELOPING THE DISTRICT HEATING SYSTEM
District heating is a key action within the sustainable energy city plan. Turin’s district heating system (DHS) is one of the most significant in Italy and Europe in terms of its size and complexity.
|Figure 2. Current and future areas of Turin DHS and thermal production plants|
The development of district heating in Turin’s urban area started in the early 1980s, and was driven primarily by the strategic directives of the Piedmont region and the province of Turin.
The system has since been extended through several expansion projects. These started with districts to the south of the city, where around 27 million m3 (Mm3) of buildings were connected) followed by the city centre, covering 12 Mm3).
At present, the network runs for 350 km and can supply 39 Mm3 of buildings – nearly 40% of the heated volume of buildings in Turin – catering for 400,000 citizens. About 84% of the grid’s users are residential and only 16% in the services sector. The generation plants supplying the system include:
- two combined-cycle gas turbines (CCGT) units in CHP production mode, each with a maximum thermal production of 260 MW;
- several backup boilers with a combined thermal capacity of 868 MW, located in three different areas of the DHS; and
- a heat storage system of 2400 m3.
The system’s characteristics are shown in Figure 2 and Table 1. The system’s current heat requirement of 1800 GWh is mainly met (79%) through CCGT plants and to a lesser extent (21%) by backup boilers. About 3% of the share produced by the CCGTs is delivered to the final users through heat storage.
Over the coming years, the Turin district heating system is due to be extended through several projects to connect new areas in the north of the city. Positive results from the storage systems at Politecnico have led to planned installations of new storage systems at all new sites.
|Table 1. Current situation (2011) and future development of the Turin DHS|
A new CHP plant has just been built to connect another 18 Mm3 of building area in the north of Turin (Torino Nord) and a backup boiler is scheduled for construction in Turin’s northeast districts (Nord-Est).
These projects will extend the urban district heating network from its current 39 Mm3 to an impressive total of 73 Mm3.
BEHAVIOUR OF THE DHS
A detailed study of the Turin district heating system’s consumption and its plants’ operation was carried out to develop a model for simulating the behaviour of the CCGT units and thermal storage tanks.
These systems can store heat at night time, when demand is minimal, and use it during the early morning demand peak. The simulation modelled performance at night time and during the early hours of the morning, when the greatest amount of energy is involved.
During the coldest periods of the year, the discharge of stored energy is not enough to meet morning demand and backup boilers have to contribute. In the milder seasons, stored heat is often enough, if delivered quickly, without requiring backup from boilers.
|A. Future developments without heat storage B. Future developments with heat storage|
|Figure 3. A comparison of primary energy consumption and the flow of thermal energy in the Turin DHS without (A) and with (B) heat storage.|
The simulation model was used to evaluate the overall performance of the system and of each generation unit (including storage); with increased thermal demand related to the connection of new city users. The new cumulative curve was calculated (see Figure 4).
Network extensions and new generation systems will deliver primary energy savings of more than 3800 GWh (2460 GWh from the completion of the current district heating system and 1380 GWh from new extensions) or a total saving of more than 1700 ktonnes of carbon dioxide emissions.
Heat storage will increase CHP production by 189 GWh/year – a 7% increase in power production – and reduce the contribution of backup boilers by 20%. This leads to a reduction of primary energy of 128 GWh and related environmental benefits. Figure 3 shows the comparison between the planned configuration with heat storage and the theoretical configuration without them.
Charging the heat storage systems requires an increase of 78 GWh in primary energy for CHP production, while producing the same power with the backup boilers would require a primary energy consumption of 205 GWh. As a result, the storage systems can reduce by 62% the primary energy needed to produce the energy that they store.
|Figure 4. Simulation of future thermal requirement of Turin DHS divided by different production plants|
On the other hand, the primary energy savings obtained through the storage systems affect by only 1% the overall DHS primary energy consumption.
With the guidelines of its Sustainable Energy Action Plan, Turin is moving towards the creation of a Smart Energy urban system.
The planned develop-ment of the district heating system will make a valuable contribution towards achieving this goal. The simulation has shown that the installation of heat storage systems optimizes the efficiency of the district heating system, reducing the heat that backup boilers have to produce to meet peak demand and increasing CHP generation. In this way, further primary energy savings can be achieved.
However, the question of how to achieve primary energy savings also has other answers beyond the increase in the thermodynamic efficiency of heat production. Reducing the heat demand is another option. There should be a mix of both strategies and a new network system paradigm, which could be called a Smart Thermal Energy Grids (STEG),
A STEG would be a means to deliver more energy efficient buildings, to use existing infrastructures in a more efficient way, to take advantage of any locally available thermal energy sources, which under normal circumstances would be wasted, to flatten the heating demand profile daily (peak shaving) and seasonally (district cooling), to use CHP and micro-CHP but also to accommodate, when possible, renewable energy sources (solar thermal, biomass, municipal solid waste (MSW), geothermal).
Gian Vincenzo Fracastoro and Alberto Poggio are with the Energy Department of the Politecnico di Torino, Torino, Italy. Email: firstname.lastname@example.org; email@example.com