Growing energy demand coupled with the need for reducing greenhouse gas emissions, thus improving the living standard in terms of human health, directly leads to the increasing use of renewable energy sources (RESs), such as solar and wind energy. Different RESs can be coupled in response to the energy crisis [
1], but a proper techno-economic feasibility study is necessary both to avoid a poor hybrid system efficiency and to obtain advantages from the economic point of view [
2]. As the sun and wind are intermittent sources, they are not always available when needed. Consequently, to meet the load request, it is necessary to integrate energy conversion and storage systems. A microgrid is a perfectly suitable solution to couple different RESs, allocating them in the same area, working either in a stand-alone or grid-connected configuration [
3]. The above-mentioned RESs’ drawbacks can be addressed and thus solved using fuel cell and electrolyser systems, which can support the plant in case of lack of electricity and produce hydrogen in case of excessive production [
4]. The latter is considered as a viable choice for the electrical energy surplus storage since it can be used as a backup system both for short and long periods. Such an outlined aspect is especially valid for places that are harsh to be electrically fed or connected with the national electrical grid, such as the ones considered in this study (i.e., telecom tower applications), where hydrogen can strongly improve the resiliency and self-sustainability of the microgrid. Although the capital expenditures (CAPEX) of hydrogen technologies are higher with respect to competing technologies, the reduced operative expenditures (OPEX) and long lifetime reliability make them a feasible solution for their use in telecom tower applications, namely the subject of the current paper [
5]. In this application field, the hydrogen-based technologies turn out to be an effective alternative to diesel generators, normally coupled with batteries in areas characterised by connection instability, in terms of sustainability [
6,
7]. Furthermore, PEMFCS may be preferrable with respect to long lasting alkaline systems due to their higher power density, efficiency and faster dynamic response [
8]. On the other hand, compared to solid oxide systems, used in [
9], PEMFCS exhibit a limited round trip efficiency, but their faster dynamic response allows their use in contexts requiring sudden power variations. One promising technology that can be coupled with PEMFCS is metal hydride (MH) storage, which is based on the reversible chemical process in which a crystal structure solid metal and the hydrogen gas are involved and both return to their original phase. Reduced space requirements, high energy density and low operating pressure make this technology very interesting in those applications, reducing personnel risks during maintenance services [
10]. Although PEMFCS can replace batteries as energy storage systems, the lack of incentives from the economic point of view may represent a limitation. It is also worth specifying that the latter mentioned drawback can also be faced and partially addressed by plant optimisation procedures as well as effective energy management strategies such as the ones proposed herein, especially in cases where the connection to the electrical grid can be expensive. Against this background, in RES-based microgrids coupled with PEMFCS, two configuration options may be investigated, islanded or grid extended. In the latter case, the grid extension is introduced, and it is defined as the distance between the nearest electric connection and the microgrid site. Ayodele T.R. and al. in [
11] analysed the topic by defining a microgrid configuration for health application in rural positions composed by PV panels, wind turbines, a fuel cell system and a hydrogen tank, finally comparing with the grid extended configuration using the break-even grid distance. The latter is here defined as the distance from the nearest network connection at which the net present cost of extending the grid is equal to the net present cost of the stand-alone system, which, in this case, is 8.81 km. This in turn implies that if the grid is far away from this distance, then the stand-alone system is a better option. Although this study is innovative for different reasons, such as the use of hydrogen as an energy storage system instead of batteries and the net present cost to study the investment in long-term periods, the model considers only the distance to determine whether the islanded microgrid is economically convenient or not with respect to the grid-extended one. Luta, D. N., et al. in [
12] directly compare the grid-extended configuration and the islanded microgrid in the region of Napier in Africa. Even in this case, a similar simplified model for the break-even distance is used, but a sensitivity analysis on the hydrogen cost is carried out. The other economic metric considered in these studies is the
LCOE, which is commonly defined as the average cost of generating one kWh of electrical energy over the lifetime of the most durable investment for the plant element [
13,
14], and, on the other hand, it is used due to its usefulness to compare different energy production methods, as in [
15], where an
LCOE-based procedure is used to obtain an economic comparison of a reversible solid oxide cell (rSOC)-based microgrid with other technologies available on the market. It is worth saying that hybridisation among energy systems results in a viable solution to reduce the latter index. In this regard, Taghavifar, H., and Zomorodian, Z. S. in [
16] propose a micro-hybrid system installation in on-grid mode (for which the grid extension is equal to 0 since the plant is already grid connected) to sellback the excess electricity and hence economise the building of a high-potential campus site as a source of income using the
SPB to analyse the economic feasibility with two scenarios of about 4.6 and 8.11 years. The integration and optimal utilisation of different accessible energy resources into a self-contained micro-grid is examined, and it is mainly focused on the
LCOE as an economic index using HOMER as optimisation tool and finding a result of about 0.5151
$/kWh. Moreover, in [
17], the connection to the electric grid is considered to obtain better economic results by scaling down the hydrogen-based system used to cover the electrical load given by a fleet of electric and hydrogen vehicles and a residential complex.
The aim of this work is to develop a versatile procedure for the model-based design of a telecommunication green power supply unit consisting of PEMFCS as main supply units and PEMELS to properly exploit available RESs. To clearly outline the wide range of applications of the designed sizing tool, two energy management scenarios leading to different plant sizing, namely charge sustaining and depleting of the HST, have been pursued. Telecom applications that exhibit non-standard geographical positions, loads and energy availability are perfectly suitable to be optimised via such tool. Furthermore, a novel contribution given by this paper is the introduction of improved SPB and LCOE economic indices, which can account for the current hydrogen price and the grid extension distance. Particularly, the latter parameter is crucial to decide whether it is economically convenient to keep a site as a remote one or connect it to the electrical grid. The other interesting innovation introduced in this paper is related to the exploitation of dynamic programming (DP) features to solve the control problem of the grid-connected microgrids, especially for the ones in which it could be advantageous to have a network connection. Indeed, the DP tool, taking as input the already optimised plant, performs a further downsizing of plant components to take full advantage of network connection while managing the power split and complying with applied energy consumption constraints. Exploiting PEMFCS and PEMELS flexibility, plant optimisation and subsequent development of year-through dynamic programming-based energy management strategies allowed achieving relevant techno-economic assessment outcomes. Such results clearly indicate to potential investors the benefit of adopting a well-designed and controlled hydrogen-based microgrid even in the short-term scenario, still characterised by relevant key components cost.
The paper is organised as follows.
Section 2 gives a description of the assumed plant and related loads. In
Section 3, a mathematical model is presented for each plant component. This allows us to introduce the optimisation procedure performed in
Section 4, in which the economic indices,
LCOE and
SPB, are detailed and the optimal sizing of the islanded microgrid elements is obtained. Then, in
Section 5, the DP routine is first introduced and then used as the control strategy for moving towards the grid-connected configuration, finding the best power split between PEMFCS, PEMELS and electrical grid itself. Finally, the main research outcomes, conclusions and follow-ups are provided in the concluding remarks.