A review of mechanical energy storage systems combined with wind and solar applications

Mechanical energy storage systems are among the most efficient and sustainable energy storage systems. There are three main types of mechanical energy storage systems; flywheel, pumped hydro and compressed air. This paper discusses the recent advances of mechanical energy storage systems coupled with wind and solar energies in terms of their utilization. It also discusses the advances and evolution in each type and compares them in terms of performance, capacity, response and utilizations. The reviewed studies exhibit all parameters that affect the performance of each storage type in which the configuration of the system has the major effective role. Choosing the suitable mechanical storage type depends on the requirements of each application such as using the flywheel for short duration applications. If long duration is needed, then it is preferred to use either pumped hydro or compressed air storage systems, knowing that the former has higher efficiency while the latter provides a faster start up. For the sake of the environment, it is recommended to use the adiabatic or isothermal compressed air storage. In all cases that combine MESSs with solar or wind energy, the series connection is preferred in order to provide stability and better control strategy

The promise and challenges of utility-scale compressed air energy storage in aquifers

Widely distributed aquifers have been proposed as effective storage reservoirs for compressed air energy storage (CAES). This aims to overcome the limitations of geological conditions for conventional utility-scale CAES, which has to date used caverns as the storage reservoirs. As a promising technology, compressed air energy storage in aquifers (CAESA) has received increasing attention as a potential method to deal with the intermittent nature of solar or wind energy sources. This article presents a selective review of theoretical and numerical modeling studies as well as field tests, along with efficiency and economic analyses, to assess the feasibility of the emerging technology. Although some field tests suggest that a large bubble could be created in aquifers to sustain the working cycles at target rates, challenges remain before the technology can be recommended for wide deployment. The geological critical safety factors affecting the gas bubble development and sustainability of operation cycles include the geological structure, aquifer depth, and hydrodynamic and mechanical properties, such as porosity, permeability, compressibility, and mineral composition. Moreover, the injection/withdrawal well configurations and oxidation reactions caused by the oxygen in compressed air should also be considered. The failed attempt of renewable energy combined with CAESA in Iowa is described and the lessons learned are summarized. Combining CAESA with thermal storage, using CO2 as cushion gas, horizontal wells or hydraulic fracturing, and man-made boundaries are proposed to improve CAESA efficiency but need further study for future applications

Varttitaseen ja sähkövarastojen vaikutukset itsenäiselle tuulivoiman tuottajalle

The popularity of intermittent renewable energy sources and electricity storages is increasing. As a response to increasing intermittent renewable energy generation, the Finnish transmission system operator Fingrid plans to introduce a 15-minute imbalance settlement period and several other changes on the Finnish electricity market in the near future. The goal of this thesis is to study the effect of the 15-minute imbalance settlement period together with other market changes and the profitability of electricity storage from the viewpoint of an independent wind power producer. The study is performed by analysing the day-ahead, intraday and balancing power markets in Germany and Finland. Also, Finnish FCR-N markets, wind power production and forecast data and production data from some Finnish wind turbines is investigated. The main findings are that balancing costs of Finnish wind power production are currently low, however, in the near future the 15-minute imbalance settlement period will encourage power producers to shift from paying balancing fees to participating actively on the intraday markets. The intraday markets will likely have a particular shape in which the first and last quarter have the highest and lowest prices and highest volumes, and the pricing will likely be unfavourable for a wind power producer. However, intraday prices could be, to some extent, predictable from day-ahead prices. Another main finding is that the current amount of wind power decreases electricity market prices only slightly, but the risk of price cannibalization might increase in the future. Wind power production in Finland seems geographically heterogeneous and the risk of cannibalization can be affected by an appropriate distribution of wind farms. The last main finding is that an electricity storage is profitable on the current hourly FCR-N market if charged occasionally with excess wind power resulting from erroneous wind power forecasts. The profitability of an electricity storage increases for a wind power producer in the next few years when the 15-minute imbalance settlement period is implemented and the share of wind power in the Finnish power system increases.Vaihtelevat uusiutuvat energianlähteet ja sähkövarastot kasvattavat suosiotaan. Vastatoimena lisääntyville uusiutuville energianlähteille Suomen kantaverkkoyhtiö Fingrid suunnittelee viidentoista minuutin taseselvitysjakson eli varttitaseen ja monen muun uudistuksen käyttöönottoa sähkömarkkinoilla lähitulevaisuudessa. Tämän työn tavoitteena on tutkia varttitaseen ja muiden sähkömarkkinauudistusten vaikutusta sekä sähkövaraston taloudellista kannattavuutta itsenäisen tuulivoimatuottajan näkökulmasta. Tutkimus toteutetaan tarkastelemalla vuorokausi-, päivänsisäisiä ja säätösähkömarkkinoita Saksassa ja Suomessa. Lisäksi Suomen FCR-N markkinoita, tuulivoiman tuotantoa ja tuotantoennustetta sekä yksittäisten suomalaisten tuulivoimaloiden tuotantodataa tutkitaan. Tutkimustulokset viittaavat siihen, että tällä hetkellä tuulivoiman tasevirheestä maksaminen on edullista, mutta lähitulevaisuudessa varttitase tulee rohkaisemaan voimantuottajia siirtymään tasevirheiden maksamisesta kaupankäyntiin päivänsisäisillä markkinoilla. Päivänsisäisillä markkinoilla tulee mahdollisesti olemaan muoto, jossa kunkin tunnin ensimmäisellä ja viimeisellä vartilla tulee olemaan korkeimmat ja alhaisimmat hinnat, ja tämä muoto tulee todennäköisesti olemaan epäsuotuisa tuulivoiman tuottajalle. Päivänsisäisten markkinoiden hinnat tulevat kuitenkin mahdollisesti olemaan jokseenkin ennustettavissa vuorokausimarkkinahinnoista. Tutkimustulokset indikoivat, että nykyinen tuulivoiman tuotannon määrä laskee vain hivenen sähkömarkkinahintoja, mutta tulevaisuudessa on riski kannibalisaatiolle. Tuulivoiman tuotanto vaikuttaa Suomessa kuitenkin maantieteellisesti heterogeeniseltä ja kannibalisaation riskiin voidaan vaikuttaa tuulivoimaloiden asianmukaisella hajauttamisella. Viimeinen tutkimustulos osoittaa, että sähkövarasto on kannattava nykyisillä FCR-N tuntimarkkinoilla, jos sitä ladataan silloin tällöin virheellisen tuulivoimaennusteen aiheuttamalla ylimääräisellä tuulivoimalla. Sähkövaraston hyöty tuulivoiman tuottajalle kasvaa lähivuosina varttitaseen käyttöönoton ja Suomen tuulivoimatuotannon kasvun johdosta

Optimal integration of compressed air energy storage system for off-grid communities

The integration of compressed air energy storage (CAES) and wind energy offers an attractive energy solution for remote areas with limited access to reliable and affordable energy sources. This thesis presents a design approach for an energy system comprising wind turbines, CAES, and diesel generators to satisfy the electricity demand in remote communities. This thesis proposes a bi-level programming (BLP) approach enabling the simultaneous optimization of the size and operation of the system while considering the interaction between them. Detailed mechanical design and configuration of the CAES system are considered, including the number and size of compressors and turbines, valves, recuperator operating conditions, etc. In contrast with conventional CAES systems, operating once a day for peak shaving, the proposed CAES system aims to mitigate wind fluctuations. Therefore, its operation is different from conventional CAES systems, and it would operate under partial load conditions most of the time, and as a result, the system's off-design modeling is also considered. The findings of this thesis indicate that the proposed system is a promising, cost-effective, reliable energy solution for remote areas, significantly decreasing the average daily total cost and CO2 emissions by 69% and 76%, respectively. Additionally, by studying the system's performance under both design and off-design conditions, it is concluded that considering the off-design conditions is critical to ensure a more realistic performance of the system as the system is less likely to utilize the CAES in low charging and discharging opportunities

Dynamic analysis and energy management strategies of micro gas turbine systems integrated with mechanical, electrochemical and thermal energy storage devices

The growing concern related to the rise of greenhouse gases in the atmosphere has led to an increase of share of renewable energy sources. Due to their unpredictability and intermittency, new flexible and efficient power systems need to be developed to compensate for this fluctuating power production. In this context, micro gas turbines have high potential for small-scale combined heat and power (CHP) applications considering their fuel flexibility, quick load changes, low maintenance, low vibrations, and high overall efficiency. Furthermore, the combination of micro gas turbines with energy storage systems can further increase the overall system flexibility and the response to rapid load changes. This thesis aims to analyse the integration of micro gas turbines with the following energy storage systems: compressed air energy storage (CAES), chemical energy storage (using hydrogen and ammonia), battery storage, and thermal energy storage. In particular, micro gas turbines integrated with CAES systems and alternative fuels operate in different working conditions compared to their standard conditions. Applications requiring increased mass flow rate at the expander, such as CAES and the use of fuels with low LHV, such as ammonia, can potentially reduce the compressor surge margin. Conversely, sudden composition changes of high LHV fuels, such as hydrogen, can cause temperature peaks, detrimental for the turbine and recuperator life. A validated model of a T100 micro gas turbine is used to analyse transitions between different conditions, identify operational limits and test the control system. Starting from the dynamic constraints defined in the related chapters, in the final part, an optimisation tool for energy management is developed to couple the micro gas turbine with energy storage systems, maximizing the plant profitability and satisfying the local electrical and thermal demands. For the modelling of the CAES system and alternative fuels, the operating constraints obtained from the initial analyses are implemented in the optimisation tool. In addition, a battery and thermal energy storage system are also considered. In the first part, a comprehensive analysis of the T100 combined with a second-generation CAES system showed enhanced efficiency, reduced fuel consumption, reduced thermal power output and increased maximum electrical power output due to the reduction of the rotational speed. The study identified optimal air injection constraints, demonstrating a +3.23% efficiency increase at 80 kW net power with a maximum mass flow rate of 50 g/s. The dynamic analysis exposed potential instabilities issues during air step injections, mitigated by using ramps at a rate of +0.5 (g/s)/s for safe and rapid dynamic mode operation. The second part explored the effects of varying H2-NG and NH3-NG blends on the T100 mGT. Steady-state results showed increased power output with hydrogen or ammonia, notably +6.1 kW for 100% H2 and up to +11.3 kW for 100% NH3. Transient power steps simulations showed surge margin reductions, especially at lower power levels with high concentrations of ammonia, highlighting the need for controlled transitions. Controlled ramps were effective in preventing extreme temperature peaks during fuel composition changes. The final chapter focused on developing an energy scheduler for different plant setups, evaluating four configurations. For a typical day of the month of April of the Savona Campus, the integration of the CAES lead to relative savings of +8.1% and power-to-H2 of +5.3% when surplus electricity was not sold to the grid. Conversely, with the ability to sell excess electricity, CAES and battery energy storage (BES) systems exhibit modest savings of +1.2% and +2.4%, respectively, while the power-to-H2 system failed to provide economic advantages