Real Case Study: Vransko municipality district heating system (Slovenia)

The test site in the Vransko municipality has been selected by ENERGETIKA in order to perform test assessment in operational environment of the sub-systems and the InDeal system for district heating system.

1. General Description

Energetika Projekt d.o.o. operates with a biomass district heating in municipality Vransko from 2003. Vransko is a small municipality in Slovenia with 2’526 inhabitants, covering an area of 5’297 hectares.
Largest natural wealth of this municipality are forests with a coverage of 77% of it area, which are mainly privately owned. 60% of the homes are heated with wood as the sole or main source of energy. This confirms that the wood is the traditional source of energy for heating of residential areas and domestic hot water preparation. Due to this configuration, a biomass district heating project has been designed in order to fulfil the needs of all the Vransko’s inhabitants. Nowadays, the power available for the connected users is 4.8MW, providing the heating and the hot domestic water throughout the whole year (8760 hours/year).
3 Boilers are used to feed this DHS are:
• 1 biomass boiler burning wood chips with a power per unit of 2MW
• 1 biomass boiler burning wood chips with a power per unit of 1.2MW
• 1 oil fuel standby boiler with a power per unit of 1.5MW
This DH can run entirely on wood chips (the oil fueled boiler is only used as a reserve).
6’450 loose cubic meter of wood chips are yearly used in average, which are supplied by surrounding farmers. Due to the use of different wood chip suppliers, both G50 and G30 types are used, with 25-35% of moisture content.
Both biomass boilers were produced in Slovenia by a company KIV d.d., a local manufacturer ensuring high efficiency by using biomass. The biggest boiler, type KIV BHH 2000, has an estimated fuel use of 4m3/h, while the smaller one, type KIV MODUL R/H 2000, uses an estimated 2.4m3/h biomass fuel. Both boilers are designed for burning biomass with relatively high humidity. The entire burning process is computerized. The wood chips are stored in a large warehouse of 830m3; and in the second – small warehouse, so called daily storage, which capacity is 370m3.
1 small CHP plant is also used with a power per unit of 120kW (wood gasification).
The global plant is located on the south of Vransko, within an area of 1’300m2. It consists in a boiler room, a large warehouse for wood chips, a small warehouse for wood chips with transport system (so called “daily storage”), a control room, offices and external manipulation space.

The Vransko DH has also a system of flat solar collectors which are integrated into the biomass system. It consists in 840m2 of solar collectors, incorporating 370kW to the DH system, helping the biomass boilers and the preparing of hot domestic water for the whole city. The connection of the solar collectors was conducted according to the principle of Low-Flow in seven receptive fields. In this way, high efficiency in solar energy is reached with a temperature drop of about 30K.
To meet the requirements, the solar system needed adequate storage tank. We installed 100m3 storage tank (3’900 MWh). Storage capacity also assist in economical operation of biomass boilers in the summer and during winter peak load.

The entire district heating net is 12’100 m long (9’050m of primary and 3’050m of secondary net pipes). The layout of this network can be found in Annex 1. It consists of preinsulated steel pipes from DN180 to DN25. Size of supply area is 1.2km2. As needs of hot water for sanitary use was required, the network has been designed with adapted high supply-return temperatures (95-65°). The supply-return temperatures of the hot network respect the fallowing rule:
Heating water temperature tolerance at each consumption point is +/- 3 ° C.
The connected users to BDH system Vransko are individual houses, multi-flat buildings, industrial building, all the larger public users (school, kindergarten, sports hall, medical center, municipality building), some shops, a restaurant, a bank, a cultural center and the post office. At the end, 189 end users are connected with exchangers according to the following diagram:

Most of the buildings connected to Vransko DHS are quite old (40+ years) with poor thermal insulation, while newer buildings (<15 years) are mostly well insulated. Buildings achieved after 2006, have to be connected to a DH according to a municipal ordinance. This kind of building is in a low-energy class (B1, B2: 15-35 kWh/m2.a).
Consumers have in recent years begun to implement partial energy efficiency of their buildings (additional thermal insulation, replacement of joinery …). Currently structure of this kind of buildings is about 5%.
In 2015, Vransko DH has produced 4’318’699kWh of thermal energy, with 319’360kWh from solar collectors. 3’586’270 kWh has been supplied to the end users (BDH system efficiency is 83.04%).

2 Current state and needs for improvement

Technology and installation of the system is 13 years old. Boilers, heating station and district heating network are reviewed and serviced regularly. Except for four minor defects (leaks) in heating network, there was no major problems.
Energetika always works in direction to increase boiler efficiency and efficiency of a whole system. Any improvement is welcomed in order to increase the efficiency, reduce energy consumption, reduce fuel consumption, or operate boiler or system optimization.