Investigating techno-economic effects and environmental impacts of energy renovation of residential building clusters on a district heating system

 

Abstract

Studying energy renovation of older residential building stocks together with the surrounding district heating (DH) system is essential to fully understand implications from a reduced district heat demand and for involved actors. This study reveals the benefit of integrating the simulation and optimization tools OPERA-MILP, IDA ICE, and MODEST to investigate thoroughly the effect of energy renovation strategies on heat load supply, building and DH system-based life cycle costs (LCCs), primary energy use, and total of direct and indirect CO2 emissions. Energy renovation considered different cluster combinations of slab and tower blocks of a stock of 343 apartment buildings in two Swedish municipalities and strategies for lowest LCC and code compliance. 

1. Introduction

The efficient use of energy in the European Union (EU) is a main topic to protect the environment, mitigate climate change, and reduce reliance on external supplies. With approx. 40% of EU’s overall energy use, the building sector is the largest single energy user and responsible for 36% of the CO2 emissions. Residential buildings represent with 76% the majority of the EU stock measured as total floor area, around 81% of their energy use corresponds to space and water heating, and half of the buildings in most EU countries are over 50 years old and built before the introduction of thermal standards, indicating a large energy-saving potential. Thus, building directives concern major renovations of existing buildings with at least cost-optimal energy efficiency levels and long-term strategies to support and accelerate the renovation of national building stocks in the EU 

2. Study description and defined cases

DH system and residential buildings in the study are sited in the municipalities of Gävle and Sandviken in fairly close proximity. These municipalities, with a population of over 130,000 inhabitants, are located in Gävleborg County in east central Sweden.

2.1. District heating system and involved actors

The DH system represents interconnected local systems from the municipalities. Existing heat supply sources consist of municipally owned plant installations for heat and electricity production and heat deliveries from industries. The municipal energy companies operate biofuel-based CHP plants with integrated flue-gas condensation and heat-only boilers or plants fueled with bio-oil, fuel oil, LPG (liquefied petroleum gas), or wood powder. Industrial heat deliveries from a pulp and paperboard mill and jointly owned industrial CHP plant fired with biofuels contain recovered process and flue-gas heat, evaporator heat, and heat from a fuel oil-fired heat-only boiler and steam condenser. Customers using district heat, among others, are the owners of the apartment buildings in the study.

2.2. Residential building clusters

The residential buildings, constructed during the time period 1965–74 as part of the so-called “Million Homes” program, include 343 multi-story apartment buildings. Buildings embody an aggregated heated area of 700,300 m2 and energy use of 97.1 GWh annually for space heating (79%) and DHW (21%). Compared to existing multi-story Million Homes buildings using DH in the municipalities, the buildings represent 86% of the stock, 76% of the heated area, and 77% of the annual energy use, and consist of clusters with 94 two-story, 160 three-story, and 89 five-story buildings, only a small number of the buildings underwent renovations with extensive investments (Renovation 20–70%), for instance, beyond maintenance with implemented EEMs, while the majority have undergone renovation with adequate investments.