Preliminary feasibility analysis of a hybrid pumped-hydro energy storage system using abandoned coal mine goafs
Introduction
Over the past 50 years (1965–2014), the world cumulatively consumed 235 billion tons of coal, 160 billion tons of oil and 97 trillion cube of gas, leading to massive carbon emissions and causing various types of environmental pollution [1]. In the framework of the Paris climate agreement, national governments had to take firm actions to increase the share of clean energy resources while reducing the usage of fossil fuel, especially coal [2], [3]. The European Union set a goal of increasing renewable energy to 32% by 2030 [4]. According to the U.S. Energy Information Administration (EIA), electricity produced by renewable energy has amounted to 19.35% of the total generated electricity and non-water renewable energy was 10.68% of that total, which far exceeded the prediction made by EIA for 2035 [5]. In China, the energy consumption will reach a peak of 6 billion tons standard coal in 2030 and the installed power generation capacity will surpass 2.8TW [6]. The share of thermal power will fall from 62% in 2017 to 50% in 2030 and clean energy resources will have more important shares, 7.37% (wind power) [7], 5.81% (solar power) [8]. Global energy is accelerating its transformation to a highly efficient, clean and diversified energy source. Renewable energy is becoming central to this global energy transition.
The growing adoption of renewable energy would increase the demand for energy storage facilities, especially large-scale energy storages. Some existing energy storage technologies, including chemical battery-based storage [9], [10], compressed air energy storage (CAES) [11], [12] and pumped hydroelectric storage (PHS) [13] are economical over various time scales, but only two, CAES and PHS are cost-effective at large temporal scales, from several hours to many days. CAES is known to require harsh geographic conditions like ensuring well impermeability to store high pressure air/gas [14], [15]. Huntorf CAES plant (1978) and McIntosh CAES plant (1991), both utilized underground salt caverns to keep compressed air and natural gas [16]. Since Alabama Electric Cooperative installed the second commercial CAES plant in McIntosh, Alabama, in 1991, there has not been any new CAES plant for more than 30 years with the issue of proper location being one of the biggest obstacles. PHS appeared to be technically more mature with lower requirements for serving as an operational site [17], [18], [19].
PHS as a promising technology to solve the imbalance of wind and solar power in electricity production has attracted a lot of attention [20], [21]. At the end of 2017, the operated PHS plants worldwide had an installed capacity of 149.95 GW. However, compared to the fast growth of renewable energy in the future, PHS plants would largely be insufficient and there will be an apparent problem of site selection [22].
Many scholars proposed to use abandoned mines in Belgium [23], [24], China [25], [26], Spain [27], Germany [28], etc., to construct PHS plants, providing a new approach for PHS site selection. Two water reservoirs with a considerable altitude difference are necessary for constructing a PHS plant. During coal exploitation, large quantities of underground caverns are formed. Using the underground space from abandoned mines as PHS reservoirs will not only avoid the need for excavation, but will also help to preserve water resources [29]. At the same time, some ground industrial complexes (buildings and living facilities) also can accommodate the equipment and staffs. As a result, utilizing abandoned mines as PHS could promote the development of clean energy and simultaneously save massive investments and time, as well as protect the environment in the region. Studies conducted on this topic include the work of Frank Winde who explored the viability of deep level gold mines in the Far West Rand gold field, South Africa, for underground pumped hydroelectric energy storage [30]. Spriet explored the possibility for using Martelange mine (Belgium) as an underground PHS plant [31]. In Germany, the Prosper-Haniel coal mine was planned to be converted into a 200 MW PHS plant, which will use 25 km of underground tunnel networks as the lower reservoir and a ground lake as the upper reservoir [28]. Javier Menéndez et al proposed to use more than 30 coal mines that will fade out in 2018 in the Asturian Central Coal Basin of Spain [27]. The results of their research confirmed that the use of underground mines as lower reservoirs for PSH was technically feasible. They all focused on the feasibility of underground tunnels, ignoring the goaf area which has a much larger storable space. It is known that the storable volume of reservoirs is essential for PHS plants, which decides the capacity to receive and regulate the excess power from grids. Distinguishing the storability of goafs and estimating the storable volume is fundamental to develop underground mine PHS plants. However, no studies in the literature have focused on this issue.
As for the coal mine goafs, Wang Qiqing et al investigated the formation conditions and mechanisms of goaf water, showing the preliminary potential of goaf serving as the lower reservoir for PHS [32]. Amin Al-Habaibeh et al analyzed the performance of using water from abandoned coal mine goafs for heating and cooling buildings, suggesting that goafs have a considerable impermeability as underground water reservoirs [33]. Gu Dazhao et al. proposed a technical approach of storage and utilization of mine water through underground reservoirs in coal mine [34]. Currently, 32 coal mine underground reservoirs has been established in the Shendong Mining Area and can cover 95% of the domestic need for water consumption [35]. Aforementioned researches demonstrate the possibility of using goafs from abandoned coal mines as PHS. Different from ordinary reservoirs, the water and air within coal mine goafs, which can be considered as porous mediums, are relatively less mobile. The non-smooth exchange of water/air with the outside would cause pressure losses and thus strongly influence the system efficiency. However, the water and air mobility within the goafs have not been addressed yet.
In this paper, we presented a new concept of PHS system using abandoned coal mine goaf, coupled with wind power and solar power (hybrid-PHS). Based on the geological condition and meteorological data from a typical region, the suitability of constructing goaf reservoirs in terms of storage capacity, saturation line during charging and discharging, reservoirs seepage and pressure loss during water-air exchange within the reservoir, was preliminarily analyzed. At last, the feasibility of using abandoned coal mine goafs as hybrid-PHS plants was discussed at a national strategic level. Our findings would help to advance the technology of using abandoned coal mines in the construction of large-scale energy storages in China, Europe and some Southeast Asian countries with a rapidly growing coal mining industry.
Section snippets
System design
Coal mines normally have several workable seams [36], [37], [38]. The two PHS reservoirs can both use underground coal mine goafs to avoid the huge amount of evaporation under the continental drought climate of the TN region. The system schematic diagram is shown in Fig. 1. The upper reservoir provides water storage capacity at a higher level. The lower reservoir is at a lower depth to ensure a suitable water head. The power room accommodating appropriate turbine-pump units and associated
Model for the PHS system
The proposed system is composed of wind turbines, a photovoltaic generator, reversible hydraulic pumps/turbines, penstocks and two reservoirs. Wind turbines and the photovoltaic generator convert clean power into electricity to meet the power demand. When electricity production is greater than electricity consumption, the surplus electricity will drive pump to elevate the water from the lower to the upper reservoir, storing the potential energy. As the electricity generated by wind and solar
Performance indicator
Except for system efficiency , some parameters, like the wind/solar power ratio , which represents the share of imported wind energy (or solar) in the total imported energy, the stored-consumed ratio , indicating the ratio of stored energy to the consumed energy, the regulating-energy density (, the regulatable energy of a unit volume for the upper/lower reservoir) representing the regulation ability of PHS, are defined.
It is obvious
Suitability potential of abandoned coal mine goaf
In this section, we will address some concerning issues related to abandoned coal mine goafs used as PHS reservoirs. Firstly, the capacity of the coal mine goaf was calculated and its usability as PHS reservoir was discussed with respect to the saturation line. Then the water leakage of the reservoirs was analyzed using the Finite Element Method. Finally, the optimal ventilation shaft dimension was determined and specific horizontal ventilation tubes were proposed for air exchange.
Role of hybrid-PHS using coal mine goafs in China
According to the above analysis, in daily operation mode the REPV of hybrid-PHS using abandoned coal mine goaf is 7.05 kW·h/m3 for wind power and 0.72 kW·h/m3 for solar power. In 2017, the electricity generated by wind and solar power was 2.70 × 1010 kW·h and 9.67 × 1010 kW·h respectively. If all the energy storage for daily regulation were to be realized through the hybrid-PHS system, at least 4.74 × 107 m3 usable underground coal mine goaf would be needed. Data from the China Coal Industry
Conclusions
In this paper, a hybrid pumped-hydro energy storage system using abandoned coal mine goafs, coupled with wind and solar power was proposed. This system regulates the water flow between two reservoirs of different altitude, convert and then store the surplus energy. Based on meteorological data in China’s Three North region, the system performance was analyzed. In terms of goaf reservoirs suitability, some essential issues, such as the storage capacity, the saturation line, the seepage and
Acknowledgements
This work was supported by the National Key R&D Program of China (No. 2017YFC0804202), National Natural Science Fund (No. 51834003, 51904039), China Postdoctoral Science Foundation (2018M633318, 2019T120809), Fundamental Research Funds for the Central Universities (2018CDQYZH0018), which are all greatly appreciated.
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