The group was given the topic viability of a piezoelectric power supply on the roadway and the topics assigned to this individual research was to investigate different methods of efficient energy storage as well as capabilities to compensate for excess energy storage. Research on the viability of a switch over system to keep the street lights powered regardless of flow of traffic was also conducted. Power conversion methods for DC to AC power for transmission to the street lights must also be investigated and implemented.
It was found that the most efficient method of energy storage would be a battery energy storage system consisting of battery modules to meet to power requirement. System control in order to ensure efficient operation of the system as a whole.
Power electronics systems must be implemented which handle the conversion from DC to AC of the battery energy source. The system control allows for management of excess energy through independently controlling different batteries at required times.
The switch over system was handled by the system control. The control system is implemented as a topology in which independent parallel connected batteries could operate at low load or even in the case when a battery has failed, therefore allowing for a redundancy system to prevent power failures and maintain power at all times. For power conversion methods an inverter was the most appropriate method, it is a common technology and is able to convert the DC voltage stored in the batteries into the AC voltage required by the street lights.
The topic of the viability of a piezoelectric power supply on the roadway was the holistic theme and the topics of research for the report was to investigate different methods of efficient energy storage as well as capabilities to compensate for excess energy storage. Research on the viability of a switch over system to keep the street lights powered regardless of flow of traffic was also conducted. Power conversion methods for DC to AC power for transmission to the street lights must also be investigated and implemented. In order for the project to be feasible an effective method of energy storage and distribution must be found, that meets all the requirements of the system. Requirements include the average power generated by the piezoelectric/km and the power needs of the street lights. The system also needs to be economical and not connected to the public grid making it completely self-sustaining.
The object of this individual research was to determine the feasibility of an energy storage system that is capable of storing all the energy obtained from the piezoelectric material, switch over functionality in order to maintain power and DC to AC power conversion methods.
Overview of Report
In the following sections the different methods of energy storage with a suggested battery storage system for large scale implementation capabilities, a switch over system with given ability of the system to sustain power supply despite fluctuations in energy generations and power conversion of DC to AC conversion capabilities in order to supply the appropriate power to the street lights.
Different methods of energy storage
A battery energy storage method was selected as the most suitable method due to how the energy generated from the piezoelectric material is already in the form of voltages, thus making for easy storage once the correct signal conditioning has been applied. The type of battery for this large-scale system chosen is the Lithium-ion battery. Since the lithium is very common material and it has low atomic mass, it has been seen as candidate of high energy and power density and high specific energy density energy storage technology. In addition, keeping voltage and temperature values of these type of batteries in the safe operation range is important.
In order handle the energy requirements a battery storage system would be required. A stationary grid-connected battery energy storage system consisting of battery storage, energy control and functional control is proposed. This system would allow for enough space to store the energy generated and control energy supply in relation to the demand, thus able to control excess energy and distribute energy at required times. A more detailed description for implementation was provided below. The system described has been derived from existing battery storage system used for solar panels.
Storage System Overview
From figure 1, the system was subdivided into three distinct sections. The first section is the battery cell connections in order to meet the correct energy requirements. Secondly, the system operation which is needed to ensure optimal control of the system, and lastly the power electronics implemented to convert the energy stored in the battery modules into the correct high voltage AC needed to power the street lights. The system control is described below.
The system control is required for reliable operation of the overall system, therefore, system control is responsible for system power flow distribution. The management of energy distributed to the street lights will be governed by factors such as energy generated per hour, current energy reserves and expected heavy load times. The system described would be capable of handling excess loads through the use of individually controlling the battery modules to accommodate the excess energy and give priority to specific battery modules that have more energy stored. The battery control subsystem is vital to this functionality since it monitors each battery module and feeds the information to the system control.
Battery Interconnection of individual battery cells to battery modules and packs is a necessary step for developing stationary lithium-ion battery based storage systems. While serial connection of cells sums up the voltage of the individual batteries to the desired module or pack voltage, parallel connection of cells will increase the usable capacity. In brief, stationary battery energy storage typically rely on groups of one to multiple parallel connected cells 6which are subsequently connected in series adding up to ≤60 V to form a battery module. Multiple modules may then be interconnected in modules. The Battery thermal control, controls temperature of the cells according their specifications in terms of absolute values and temperature gradients within the pack. Cell operation and aging are strongly affected by temperature. Temperature variations within the battery pack may lead to unbalanced current flow and increased aging. As such, the battery control serves the functionality of the battery not only in terms of safety but also for enabling a long battery lifetime. A deep cycle battery was selected for its longevity. The model selected has a rated voltage of 52 V therefore, there was no need for batteries in series and thus can be arranged in parallel in order to meet the average power generated per km of 200kWh, the average power generated per km was provided by the electronic engineer in charge of power generation.
The power electronics converts the power flow between street lights and battery, and the required control determine the correct voltage that the street lights require.3 Switch over system Figure 2 gives an overview of variants of grid connection topologies battery energy storage that typically consist of multiple battery packs and inverter units, which together add up to the total system energy and power. Power electronics units can be either installed dedicated to each battery pack or the battery packs can be connected in parallel to a common DC-bus.
A clear advantage of the dedicated pack to inverter connection topology is that the power per battery pack can be individually controlled. Each unit is independent, which enables the handling of multiple grid applications at a time, in parallel. Each string still follows only a singular application request. Such a topology would allow for a switch over system since, each battery could be controlled independently with regard to the other batteries. A possible solution would be to have a separate branch of batteries with the sole purpose of operating when there was low load, thus requiring fewer number of batteries and resulting in more efficient use of the energy generated. Another benefit would be reliability, as failure of a single power electronics unit or a battery pack does not directly harm other unit’s operation and having a parallel connection topology does not necessarily lead to loss of power immediately.
In order to convert the DC voltage stored in the batteries to AC, an inverter is required. Inverters use components to convert the constant DC voltage to a time varying sinusoidal AC voltage. Four different topologies for the energy stored system are proposed, are shown in figure 3. For optimal efficiency a technology generally used for solar panel may be incorporated, which is MPPT or Maximum Power Point Tracking. It is an algorithm included in charge controllers used for extracting maximum available power from solar panels which may be adapted for the use of piezoelectric material.
Discussion The battery storage system was selected for its modular design and large scale capabilities. These traits allow for future expansions and ease of repair since there would be no single point of failure. To ensure proper control of the flow of power. A control sub-system would need to be implemented to monitor and manage power consumption to all enough energy reserves at off peak times. Inverter provide an efficient solution to power conversion from the battery to street lamps.
The most efficient method for storage of energy was lithium-ion battery energy storage system. With enough capacity to accommodate the 200kWh energy generated with 50 deep cell batteries in a parallel connection for control and redundancy purposes. The average cost of such a system would be above R2000 000 per module but would be completely self-sustaining from the public power grid. The amount for the power system does exceed expectations making it more expensive per km than redoing the road. A fail-safe system could be implemented through the use of control systems, by monitoring batteries and power flow. The power could be maintained even at low load times and excess energy could also be accounted for and stored correctly and with the use of inverters the street lights would have received the correct voltages required in AC power supplies. The system proposed would provide for efficient energy storage and distribution.
Due to the large cost of the battery energy storage system, since the cost of deep cycle lithium battery are high. More funding would be required for energy storage and distribution to be feasible. This may be a long term solution given a large up-front investment and proper maintenance of the entire system.