Research on Programming and Electronics for Aalto Explorer (Part 4)

Energy consumption for these underwater robots is another cause for concern. Many AUVs (autonomous underwater vehicles) aren’t equipped with efficient energy-storage systems, leading to energy dissipation after a certain length of time.

Aalto Explorer includes a floating module that harvests solar energy from lithium batteries to operate the unit – the floating module and the ROV.

This article continues the series of research on Programming and Electronics for AE: Power Management

5. POWER MANAGEMENT

5.1. Solar Photovoltaic System – Energy harvesting systems.

Designing an efficient solar harvesting system requires an in-depth understanding of several factors. Solar energy supply is highly time-varying and may not always be sufficient to power the embedded system.

The harvesting components, including solar panels and energy storage elements, such as batteries or ultracapacitors, have different voltage-current characteristics. In order to maximise harvesting efficiency, these characteristics must be compatible with each other as well as with the energy requirements of the system.  

Further, battery non-idealities, such as self-discharge and round-trip efficiency, directly affect energy usage and storage decisions.

Design Solar PV System

The major components in designing a solar PV system include:

  • PV module – converts sunlight into DC electricity.
  • Solar charge controller – regulates the voltage and the current coming from the PV panels which then go to the battery, prevents the battery from overcharging and prolongs battery life.
  • Inverter – converts DC output of PV panels or wind turbines into clean AC current to be used for AC appliances or fed back into the grid line.
  • Battery – stores the energy supply for electrical appliances when demanded.
  • Load – is electrical appliances that are connected to the solar PV system.

Solar PV system sizing

a) Determine the power consumption demands

The first step in designing a solar PV system is to find out the total power and energy consumption of all loads that need to be supplied by the solar PV system.

b) Size the PV modules: get total watt-peak rating

Different sizes of the PV modules will produce different amounts of power. To find out the sizing of PV module, the total peak watt that they produce is needed. The peak watt (Wp) produced depends on the size of the PV module and the climate of the site location. We have to consider the “panel generation factor” which varies in each site location.

The calculation result is the minimum number of PV panels for desired effect. If more PV modules are installed, the system will perform better and the battery life will be improved. If fewer PV modules are used, the system may not work at all during cloudy periods and the battery life will be shortened.

c) Inverter sizing

An inverter is used in the system if AC power output is needed.

d) Battery sizing

The battery type recommended for using in solar PV system is deep cycle battery.

Deep cycle battery is specifically designed to be discharged at low energy level and rapidly recharged or to conduct a cycle of charged and discharged day after day for years. The battery should be large enough to store energy sufficiently for operating the appliances at night and on cloudy days.

Battery Capacity (Ah) = Total Watt-hours per day used by appliances x Days of autonomy (0.85 x 0.6 x nominal battery voltage).

Thus, the choice of battery chemistry for a harvesting system depends upon its power usage, recharging current, and the specific point on the cost-efficiency trade-off curve that a designer chooses.

There are four types of rechargeable batteries in common use:

  • Nickel Cadmium (NiCd),
  • Nickel Metal Hydride (NiMH),
  • Lithium based (Li+1), and
  • Sealed Lead Acid (SLA).

SLA and NiCD batteries are less common because they have relatively low energy density. The latter suffers from the temporary capacity loss caused by shallow discharge cycles – in other words, the memory effect.

The choice between NiMH and Li+ batteries involves several trade-offs:

  • Li+ batteries are more efficient than NiMH,
  • Li+ have a longer cycle lifetime,
  • Li+ involve a lower rate of self-discharge.
  • Li+ are more expensive, even after accounting for their increased cycle life.
  • Li+ batteries require a more significantly complicated charging circuit.
  • Charging the Li+ batteries at very low rates is often not possible due to the charging acceptance issues,
  • Li+ are known to degrade if subjected to deep discharge cycles.

Battery aging due to the charge-discharge cycles, for example:

  • NiMH batteries (when subjected to repeated 100% discharge cycles) yield a lifetime of around 500 cycles. At the final cycles, the battery will still deliver around 80% of its rated capacity.
  • This means that the battery will have only 80% of the capacity of a new battery.
  • The residual capacity is significantly higher if the battery is only subjected to shallow discharge cycles.
  • At the rate of one discharge cycle per day, the battery will last for several years before its capacity depletes to zero.

e) Solar charge controller

The solar charge controller is typically rated against Amperage and Voltage capacities. The team must select the solar charge controller that matches the voltage of PV array and the batteries. After that, the type of solar charge controller must be identified to make sure it is right for your application. It is also important to ascertain that the solar charge controller has enough capacity to handle the current from the PV array.

For the series charge controller type, the sizing of the controller depends on the total PV input current which is delivered to the controller. Besides, the size also depends on the PV panel configuration (series or parallel configuration).

According to the standard practice, the sizing formula of the solar charge controller is to take the short circuit current (Isc) of the PV array and multiply it by 1.3.

Solar charge controller rating = Total short circuit current of PV array x 1.3

5.2. Stand-alone Solar Power Systems, Stand-alone PV Systems and Off-grid Power Systems

The DC power generated by these systems is stored in the batteries and is converted to AC power for household or commercial use.

The examples of the components:

  • 170 x Solarmax LEC-3024 30 Wp solar array (5×34)
  • 1 x SOLARCON SCP-12060 charge controller with data logger
  • 20 x 200 Ah maintenance free battery

Harvesting circuit design

The core of the harvesting module is the harvesting circuit which draws power from the solar panels, manages energy storage, and routes power to the target system.

The most important consideration in the design of this circuit is to maximize efficiency and there are several aspects that can influence this. Solar panels have an optimal operating point that yields maximal power output. The harvesting circuit should ensure operation at (or near) this maximal power point, which is done by clamping the output terminals of the solar panel to a fixed voltage.

Since the maximal power point changes throughout the day (i.e., as the incident radiation changes), a maximal power point tracker (MPPT) circuit can be used to continuously track and operate at the optimal point. However, the commercially available MPPT ICs are designed for high power applications such as the energy-consuming solar-based water heaters, precluding their use in a low power, solar harvesting, embedded system.

A DC-DC converter is often used to provide a constant supply voltage to the embedded system. The choice of DC-DC converter depends on the operating voltage range of the particular battery used, as well as the supply voltage required by the target system. If the required supply voltage falls within the voltage range of the battery, a boost-buck converter is required, because the battery voltage will have to be increased or decreased depending on the state of the battery. However, if the supply voltage falls outside of the battery’s voltage range, either a boost converter or a buck converter is sufficient, which significantly improves the power supply efficiency.

Although the specialised battery charging ICs are available, they are designed to regulate charging at significantly higher currents (e.g., wall chargers) than the few tens of mA provided by a small solar panel. They are also inefficient (if still operable) at such low currents.

Finally, since the primary goal is to efficiently harvest and utilise every precious mW of power provided by the solar panel, it is desirable to make the harvesting circuit as application- and system- specific as possible. For example, the team must build a harvesting circuit that works with several solar panels, and avoid poor decisions in selecting charges like NiCd, NiMH, and Li+ batteries because of the efficiency loss.

Energy measurement

To enable informed decisions concerning harvesting power management, the harvesting module should have enough energy measurement capabilities. Low-power battery monitor ICs can be used to manage this function. The target system should be able to query the harvesting module for data on instantaneous power being provided by the solar panels and the battery terminal voltage. In addition, the harvesting module should be able to learn the solar power availability pattern, and build and train a power macro-model that provides information about future power arrival.

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