for cogeneration of heat. Now, combined cycle gas turbines designs are beginning to be widely used as well. Serbia In Serbia, district heating is used throughout the main cities, particularly in the capital, Belgrade. The first district heating plant was built in 1961 as a means to provide effective heating to the newly built suburbs of Novi Beograd. Since then, numerous plants have been built to heat the ever-growing city. They use natural gas as fuel, because it has less of an effect on the environment. The district heating system of Belgrade possesses 112 heat sources of 2,454 MW capacity, over 500 km of pipeline, and 4,365 connection stations, providing district heating to 240,000 apartments and 7,500 office/commercial buildings of total floor area exceeding 17,000,000 square meters. Slovakia Slovakia's centralised heating system covers more than 54% of the overall demand for heat. In 2015 approximately 1.8 million citizens, 35% of the total population of Slovakia, were served by district heating. The infrastructure was built mainly during the 1960s and 1980s. In recent years large investments were made to increase the share of renewable energy sources and energy efficiency in district heating systems. The heat production comes mostly from natural gas and biomass sources, and 54% of the heat in district heating is generated through cogeneration. The distribution system consists of 2800 km of pipes. Warm and hot water are the most common heat carriers, but older high-pressure steam transport still accounts for around one-quarter of the primary distribution, which results in more losses in the system. In terms of the market structure, there were 338 heat suppliers licensed to produce and/or distribute heat in 2016, of which 87% were both producers and distributors. Most are small companies that operate in a single municipality, but some large companies such as Veolia are also present in the market. The state owns and operates large co-generation plants that produce district heat and electricity in six cities (Bratislava, Košice, Žilina, Trnava, Zvolen and Martin). Multiple companies can operate in one city, which is the case in larger cities. A large share of DH is produced by small natural gas heat boilers connected to blocks of buildings. In 2014, nearly 40% of the total DH generation was from natural gas boilers, other than co-generation. Sweden Sweden has a long tradition for using district heating (fjärrvärme) in urban areas. In 2015, about 60% of Sweden's houses