Contents lists available at ScienceDirect Renewable and Sustainable Energy Reviews journal homepage: www.elsevier.com/locate/rser Worldwide application of aquifer thermal energy storage – A review Paul Fleuchaus a, ⁎ , Bas Godschalk b , Ingrid Stober a , Philipp Blum a a Karlsruhe Institute of Technology (KIT), Institute of Applied Geosciences (AGW), Kaiserstr. 12, 76131 Karlsruhe, Germany b IF Technology BV, Velperweg 37, 6824 BE Arnhem, The Netherlands ARTICLE INFO Keywords: Underground thermal energy storage Geothermal energy Renewable energy Seasonal thermal energy storage Heating and cooling ABSTRACT To meet the global climate change mitigation targets, more attention has to be paid to the decarbonization of the heating and cooling sector. Aquifer Thermal Energy Storage (ATES) is considered to bridge the gap between periods of highest energy demand and highest energy supply. The objective of this study therefore is to review the global application status of ATES underpinned by operational statistics from existing projects. ATES is particularly suited to provide heating and cooling for large-scale applications such as public and commercial buildings, district heating, or industrial purposes. Compared to conventional technologies, ATES systems achieve energy savings between 40% and 70% and CO 2 savings of up to several thousand tons per year. Capital costs decline with increasing installed capacity, averaging 0.2 Mio. € for small systems and 2 Mio. € for large ap- plications. The typical payback time is 2–10 years. Worldwide, there are currently more than 2800 ATES systems in operation, abstracting more than 2.5 TWh of heating and cooling per year. 99% are low-temperature systems (LT-ATES) with storage temperatures of < 25 °C. 85% of all systems are located in the Netherlands, and a further 10% are found in Sweden, Denmark, and Belgium. However, there is an increasing interest in ATES technology in several countries such as Great Britain, Germany, Japan, Turkey, and China. The great discrepancy in global ATES development is attributed to several market barriers that impede market penetration. Such barriers are of socio-economic and legislative nature. 1. Introduction The global community has to face a paradigm shift towards a sus- tainable energy supply to keep the increase in the global average temperature to within 2 °C above pre-industrial levels. While the share of renewables in the power generation sector increases continuously, less attention is paid to the decarbonization of the heating and cooling sector. In 2015, heating and cooling accounted for half of the total world final energy consumption, with three-quarters produced from fossil fuels. The share of modern renewable technologies is currently estimated at only 8% [1]. At the same time, global energy consumption for heating and cooling is expected to further increase with rising prosperity, population growth, and climate change. According to IPCC (Intergovernmental Panel on Climate Change), power consumption for air conditioning alone is expected to rise 33-fold by 2100 [2]. To achieve the climate change mitigation targets, increasing attention has to be paid to the decarbonization of the thermal energy sector. The key challenge of increasing the share of renewables in the heating and cooling sector is attributed to the seasonal offset between thermal energy demand and supply. To tackle this seasonal mismatch, the idea of Thermal Energy Storage (TES) has attracted increasing at- tention [3]. The selection of an appropriate storage method depends on several factors such as storage capacity, storage duration, and supply and demand temperature [4,5]. Underground Thermal Energy Storage (UTES) is a sensible TES method, characterized by high storage effi- ciencies [6,7] and high storage capacities and is therefore the preferred choice for long-term TES. The most popular sensible seasonal UTES techniques are illustrated in Fig. 1. UTES can be further subdivided into open-loop or closed-loop systems. In open-loop systems, also referred to as Aquifer Thermal Energy Storage (ATES), sensible heat and cold is temporarily stored in the subsurface through injection and withdrawal of groundwater [8–10]. Closed-loop systems are more or less independent of the perme- ability of the subsurface and are called Borehole Thermal Energy Storage (BTES). In Tank Thermal Energy Storage (TTES), Pit Thermal Energy Storage (PTES), and Cavern Thermal Energy Storage (CTES), https://doi.org/10.1016/j.rser.2018.06.057 Received 14 November 2017; Received in revised form 19 April 2018; Accepted 24 June 2018 ⁎ Corresponding author. E-mail address: paul.fleuchaus@kit.edu (P. Fleuchaus). Abbreviations: AR, Artificial Recharge; ATES, Aquifer Thermal Energy Storage; BTES, Borehole Thermal Energy Storage; CCS, Carbon Capture and Storage; CTES, Cavern Thermal Energy Storage; ECES, Energy Conservation through Energy Storage; GHG, Greenhouse Gas; GSHP, Ground Source Heat Pump; HT, High Temperature; HVAC, Heating, Ventilation, and Air Conditioning; IEA, International Energy Agency; IPCC, Intergovernmental Panel on Climate Change; LT, Low Temperature; PTES, Pit Thermal Energy Storage; TES, Thermal Energy Storage; TRL, Technical Readiness Level; TTES, Tank Thermal Energy Storage; UTES, Underground Thermal Energy Storage Renewable and Sustainable Energy Reviews 94 (2018) 861–876 1364-0321/ © 2018 Elsevier Ltd. All rights reserved. T