District heating to replace an Electrical Installation (Mechanical Project)

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This project has been developed at the company Gavlegardarna. The company owns a large part of the buildings of Gävle and two of them are the objective of the project. Gavlegardana is highly concerned about the environment; for this reason, they cooperate on the subject with the energy management from their technical department.

Gävle is one of the Swedish cities where the DH (district heating) network is distributed, arriving to most of the dwellings, industries and commercial buildings. As DH uses environmentally friendly sources of energy,Gavlegardana is introducing it in its buildings.

Electrical radiators and boilers were installed in the buildings when the price of electricity was more affordable than nowadays. The price of the electricity can be considered 1,23 SEK/kWh while the DH price is 0,45 SEK/kWh.

Consequently, this is another reason why the objective of the company at the present time is to replace electrical space heating systems by means of district heating.

The energy balance of the buildings is analysed in order to study their current energy situation. This entails the consideration of heat gains and losses involved. The heat gains of the building are the heat from solar radiation which arrives at the building trough the windows, the heat internally generated (by persons, lighting and other devices) and the heat supplied. The heat losses are composed by the transmission trough walls and windows, the infiltrations, the heat used for hot tap water and the ventilation losses.

An important part of the work required to calculate the energy balance has consisted of the collection and organization of all the data (areas, types of material, electrical devices, lighting, number of employees, opening hours…).This data comes from the drawings of the buildings provided by the company and from the information gathered during the visits to the installation. In addition, the ventilation flows were measured in-situ using the tools provided by Theorells.

Gavle Energi, the DH distributor company, has been contacted in order to fixthe cost and other details related to the district heating connection. The heat exchanger models, selected from Palm at System AB, are TP20 for Building A and TP10 for Building B. TP20 provides 100 kW of heating and 0,4 l/s of hot tap water and TP10 provides 50 kW and 0,31 l/s respectively. The capital cost is 187500 SEK which includes the heat exchangers and the connection cost.

As the secondary circuit is not currently installed because the existing system is composed by electrical radiators, the installation of the piping network in the building has been designed. The radiators’ power is calculated taking into account the need of heat in each room which is estimated as the transmission losses. This need of heat calculated is higher than the energy currently supplied which means that the thermal comfort is not achieved in all the rooms of the buildings.

In spite of using more energy for space heating, the change of heat source entails a lower energy cost per year. The selected radiators are from Epecon and the investment cost (including the installation) is 203671 SEK. The brand of the selected pipes is Broson and the investment cost of the total piping system is 66000 SEK.

The initial investment of the new installation is 457171 SEK, considering the DH connection, heat exchangers, radiators and pipes. If the initial investment is totally paid in cash by the company the payback will be fulfilled in 6 years. In case of borrowing the money from the bank (considering an interest rate of 5%), two possibilities can be considered: paying back the money in annual rates over 15 years or 30 years of maturity. The paybacks are 11 and 8 years respectively.

After designing the DH piping system in the buildings, estimating the total costs of the investment and studying the project’s feasibility by suggesting different payment options, some possible energy savings are recommended.

The first of the options refers to the transmission losses trough the windows whose values’ are considerably high. Using a glass with a lower U-value, these losses can decrease until 66% (with triple glass windows). Consequently, the power required for space heating can also be reduced until 26%.

Regarding the ventilation, rotating heat exchangers are currently used, which entails the problem of smells mixture detected by the users of the buildings. By changing them with flat-plate heat exchangers, the problem is solved and the efficiency is increased from 66% to 85%. The new heat exchanger cost is340387 SEK and it has a payback of 10 years.
Source: University of Gävle
Author: Serra Ramon, Lourdes | Montañes Asenjo, Alba

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