Flywheel

A flywheel is a device that stores kinetic energy by accelerating a rotor witli high moment of inertia to very high speeds. It maintains the energy in the form of rotational kinetic energy. When desired, the flywheel is engaged to the generator to convert back the stored energy at rates beyond the ability of the original energy source [10]. Flywheels are useful because they do not require very much space to operate and have fairly low maintenance. However, due to the capital cost, flywheels are mainly used in commercial and macro-scale systems. In a research conducted by Prodromidis and Coutelieris [13], simulations were carried out to test the economic and technical feasibility of using flywheel-integrated renewable energy systems. The purpose of the research is to assess if flywheels could be integrated into everyday renewable energy systems. It should be noted that the simulations presented are based on the load for a typical house located on Naxos island due to its high renewable energy sources; the system is assumed to be used simultaneously with solar and wind energy; and finally, a stack of batteries was used to support the flywheel [13]. The authors propose hooking a flywheel to a DC-AC converter and, in turn, a battery [13]. To achieve the most reliable results, the authors developed six different scenarios to test (Table 6.1). The operational characteristics are presented in Table 6.2.

The most noticeable bit of information available from Table 6.1 is the reduction of batteries needed for the setups involving the flywheel compared with their original setup without the flywheel. Coming directly from this information, in Table 6.2 one can see how efficient each flywheel and battery combination will be. There are a lot of factors that can be taken from this table, with arguably the most important being the roundtrip efficiency. Being able to maintain more than 75% of the stored energy is an impressive feat. It should also be noted how all of the combinations have a long lifetime. A comparison of costs and the future of flywheel storage systems is also conducted. The authors write “it is observed that although the initial cost of the systems with simple batteries are much lower, systems combining flywheels can be competitive because the NPC (Net Present Cost) of the different systems are

TABLE 6.1

List of Scenarios Simulated by Prodromidis and Coutelieris to Evaluate Effectiveness of Integrating Flywheel into Wind/Solar Systems for a Desirable Load of 7,775 kWh

vision batteries

TABLE 6.2

Characteristics of the Scenarios Presented in Table 6.1

Source: Prodromidis, G.N. and Coutelieris, F.A., Renew. Energy, 39, 149-153, 2012.

equivalent. ... Finally, this innovative method of energy storage shows that flywheel systems could be commercialized in the near future” [13]. From the information, it can be seen that there are several advantages to using a flyw'heel-integrated renewable energy system. By adding the flywheel, the total number of batteries needed dropped in each setup (Table 6.1). The flyw'heel and battery tandems proposed in this concept yield some of the highest efficiency values found in the complete review of the present chapter. This combination of factors show's that in the right situation, a flywheel-integrated system could be an efficient method of energy storage. It should also be noted that the research by Prodromidis and Coutelieris was done in a specific location that had an abundance of the needed natural resources [13]. More work should be presented to find the effect of location. Some of the disadvantages of using a flywheel are the safety issues that arise. When running, a flywheel has rapidly rotating parts. This could be fatal if a part was pulled into the wheel, or if failure were to occur. Another issue that can arise is that flywheels are limited by their size. This could cause issues when trying to design a stand-alone flywheel system.

Super Capacitors

Super capacitors (SC) are electromechanical capacitors w'ith very high-power density (i.e., ability to release energy at very high rates [14]), while their energy density is low. SCs are similar to batteries but serve the opposite purpose. Batteries have a high energy density; however, their discharge rate is significantly smaller than SCs. SCs are also useful for their life cycle because they can go through many cycles before degradation occurs. The main issue w'ith SCs is being very costly relative to other technologies. Considering the low energy density of each SC, it is easy to notice how rapidly the implementation costs would increase if they are used in conjunction with renewable energy harvesters, for instance, a wind farm. Based on the relevant knowledge, SCs would be most useful in small-scale systems where many are not needed, such as biomedical sensors powered via piezoelectric materials.

Batteries

Batteries are devices that use a chemical process to generate electricity. High density is one of the most outstanding characteristics of batteries. Batteries also have a large life span. They can be loaded and unloaded over a large number of cycles. Although batteries offer major advantages, there are also several negative aspects associated with them. Batteries can be affected by temperature. This limits the applications and locations they can be used. Batteries are also not economical because they can be expensive to purchase in large quantities for macro-scale plants and to dispose of. The disposal issue is not only costly but also bad for the environment. So, although batteries can be useful for certain applications, the overall impact must be addressed before a decision is made.

Although it is easy for one to simply consider the negative qualities of batteries and assume there are much better options to consider for energy storage than them, batteries remain the most used storage methods when integrated with other forms of renewable energy-generation devices [15]. Nearly every research, if not all, that was reviewed for the present chapter proposes a new or improved method of alternative energy storage that used batteries as the main or backup storage medium.

According to Tiwari et al. [15], battery storage offers outstanding techno-economic advantages leading to dominance among all available energy storage technologies. Tiwari et al. [15] compared batteries to other energy-storage systems on the premise that the draining of conventional fuels and climate change has warranted a need for the reliance on renewable energy for main stream power-generation systems [15]. It is then proposed to integrate a wind-battery system with thermal generators, which are actually capable of meeting the intermittency demand alone [15]. According to their economic analysis, substituting the thermal generator system with the integrated thermal-wind-battery system leads to daily savings of nearly $66,600 [15]. The detailed cost data for the thermal generator system and the integrated thermal wind-battery system are presented in Tables 6.3 and 6.4, respectively. Although the efficiency values are not provided, the improvement of using a battery-integrated system can be seen in the economic saving shown in these tables.

The validity of using batteries in conjunction with a renewable energy-generation configuration is further confirmed by a research conducted at the University of Nottingham [16]. This study demonstrates the necessity of having better control of the amount of power generated from renewable-based power generators to ensure

TABLE 6.3

Cost for the Generator Only System Hours of

Source: Tiwari, S., et al., hit. J. Renew. Energy Res., 8, 692-701,2018.

TABLE 6.4

Cost for Thermal Generation with BES-Integrated WES

Source: Tiwari, S., et al„ Int. J. Renew. Energy Res., 8,692-701, 2018.

the demand is met, rather than having a nonstop production at maximum amount of available power, which is actually intermittent. This requires storage-integrated harvesters to store energy and then use it to overcome any fluctuations that arise from intermittency. Such fluctuations cause overcharging and fines that are occurring from both consumers and wind farm operators. Batteries are stated as “perhaps the most versatile than any of the storage devices [to overcome the intermittency and unpredictability issues from renewables] as they offer desirable characteristics for wide ranges of applications and are generally cheaper in most cases” [16]. Batteries are arguably the most developed of all energy storage systems. Although there are so many implementations that batteries can be used in, they can be expensive to purchase and set up, and disposal is horrible for the environment. The pros and cons should definitely be weighed before using a battery-integrated storage system.

Buoyancy-Based Energy Storage

The offshore buoyancy-based storage, proposed by Bassett et al. [17], uses surplus electrical power to run a motor-driven pulley system to pull down a buoyant object through an open body of water. When desired, the object will be released to travel back to the surface under the buoyancy force. The linear motion of the object will then be converted into a rotary motion to drive a generator and recreate the required electrical power. One of the most attractive aspects of this system is the scalability factor. Because of the systems setup, the primary elements needed for increased storage capacity are air and water, unlike chemical-based batteries [17]. The proposed concept has the potential storage capabilities on the magnitude of Gigawatt-hour, which is a level currently available with the use of hydro pumped and compressed air energy storage [17]. The largest stated issue is the cost of the system because it needs expensive offshore below-surface constructions requiring divers or robotic-operated vehicles. Such constructions are several times more expensive than terrestrial constructions. When developing this storage system, one of the important aspects to consider was the hydrodynamic drag [17]. As seen in Table 6.5, different drag losses are listed along with the efficiency ranges. The authors next calculated the roundtrip efficiency, which is arguably the most important aspect of the entire concept. By taking into account the efficiency of motor, generator, charge, discharge, and pulley, the roundtrip efficiency can be calculated as:

All the efficiency values are standardly used or provided. The efficiency of the system can be estimated as 83%, since: r_round,_rip = 0.97 x 0.95 x 0.97 x 0.97 x 0.96 = 0.83.