Load Profiles, Energy Demand, System Voltage and Autonomy for SS 1

Load Profile

Performance Objectives

By the end of the lesson, students should be able to:

1. discuss the meaning of load profile;
2. explain the terminologies in load profile;
3. calculate energy demand of a given facility, system voltage and days of autonomy.

Meaning of Load Profile

A load profile is a graphical representation of the electrical power demand of a facility over a specific period, typically twenty-four hours.

In solar PV installation and maintenance, understanding the load profile is a fundamental step in system design. It shows not only how much total energy a building consumes, but also when that energy is consumed. Since solar panels generate electricity only during daylight hours, knowing the timing of energy consumption helps installers determine how much energy must be stored in batteries for night-time use and how much can be used directly during the day.

A typical residential load profile in Nigeria shows distinct peaks in the morning and evening. In the morning, power usage rises as occupants prepare for school or work, using lighting, water pumps, and kitchen appliances. During the afternoon, the load drops significantly as the house is often empty. In the evening, the load peaks again as occupants return home and turn on televisions, standing fans, lighting, and other entertainment systems.

Observe the diagram below showing a typical daily load profile for a residential building.

Terminologies in Load Profile

To design and install a solar PV system successfully, several key technical terms related to load profiles must be understood. These terms form the basis of all sizing calculations.

Energy Demand

Energy demand is the total amount of electrical energy consumed by all appliances in a facility over a specific period, usually measured in Watt-hours (Wh) or Kilowatt-hours (kWh) per day.

While power (measured in Watts) indicates the rate at which an appliance consumes electricity at any single instant, energy demand accounts for both the power rating and the duration of time the appliance is switched on. For example, a 100-Watt television operated for 5 hours has an energy demand of 500 Watt-hours.

System Voltage

System voltage is the nominal direct current (DC) operating voltage of the solar PV system’s battery bank, charge controller, and inverter input.

Solar PV systems are configured to operate at specific nominal DC voltages to ensure safety, efficiency, and compatibility of components. The most common nominal system voltages used in off-grid solar installations are 12 Volts (V), 24 Volts (V), and 48 Volts (V). Selecting the correct system voltage prevents high current flow through the cables, which reduces power losses and minimizes the required thickness of the copper wiring.

Days of Autonomy

Days of autonomy is the number of consecutive days that a fully charged battery bank can supply power to the connected loads without receiving any charging current from the solar panels.

This parameter is crucial for system reliability. In Nigeria, weather conditions vary, and there are periods of heavy cloud cover, harmattan dust, or prolonged rainfall when solar panels cannot produce enough electricity to charge the batteries. Days of autonomy ensure that critical appliances continue to run during these periods of poor weather.

Calculating Energy Demand

Calculating the daily energy demand of a facility requires identifying every electrical appliance, its power rating in Watts, and the number of hours it runs each day. The power rating of an appliance is typically found on its specification tag or label, which is usually located on the back or bottom of the device.

Observe the image below showing a typical appliance power rating tag.

The formula for calculating the daily energy demand of a single appliance type is:

$$\text{Daily Energy Demand (Wh)} = \text{Power Rating (W)} \times \text{Quantity} \times \text{Daily Operating Hours (h)}$$

To find the total daily energy demand of the entire facility, the energy demands of all individual appliance types are added together.

Worked Example 1

A small office in Enugu has the following electrical appliances:

1. 5 LED bulbs, each rated at 12 Watts, used for 10 hours per day.
2. 2 standing fans, each rated at 55 Watts, used for 8 hours per day.
3. 1 desktop computer, rated at 150 Watts, used for 6 hours per day.
4. 1 small refrigerator, rated at 120 Watts, used for 24 hours per day (with a compressor duty cycle of 50%, meaning it runs for an effective total of 12 hours per day).

Calculate the total daily energy demand of the office.

Solution:

The calculations are structured in the table below:

The total daily energy demand of the office is $$3,820\text{ Watt-hours (Wh)}$$ (or $$3.82\text{ Kilowatt-hours (kWh)}$$).

Calculating and Selecting System Voltage

The nominal DC system voltage of a solar PV installation is selected based on the total daily energy demand of the facility. Selecting an appropriate system voltage keeps the system current within safe and manageable limits.

The standard industry guidelines for selecting system voltage based on daily energy demand are:

1. For daily energy demands up to $$1,200\text{ Wh}$$, a $$12\text{V}$$ system voltage is selected.
2. For daily energy demands between $$1,200\text{ Wh}$$ and $$2,400\text{ Wh}$$, a $$24\text{V}$$ system voltage is selected.
3. For daily energy demands above $$2,400\text{ Wh}$$, a $$48\text{V}$$ system voltage is selected.

Worked Example 2

Determine the appropriate nominal system voltage for the office described in Worked Example 1, which has a total daily energy demand of $$3,820\text{ Wh}$$.

Solution:

The total daily energy demand is $$3,820\text{ Wh}$$.

According to the selection guidelines, since $$3,820\text{ Wh}$$ is greater than $$2,400\text{ Wh}$$, the appropriate nominal system voltage for this facility is $$48\text{V}$$.

Calculating Days of Autonomy

Days of autonomy is a design decision based on the geographic location of the installation and how critical the electrical loads are. In Nigeria, typical values are:

1. $$1\text{ to }2\text{ days}$$ of autonomy for standard residential homes.
2. $$3\text{ to }5\text{ days}$$ of autonomy for critical facilities such as hospitals, security systems, and telecommunication stations.

To calculate the total energy storage capacity required in the battery bank to satisfy the days of autonomy, the daily energy demand is multiplied by the number of days of autonomy.

The formula is:

$$\text{Total Storage Energy (Wh)} = \text{Daily Energy Demand (Wh)} \times \text{Days of Autonomy}$$

Worked Example 3

A rural health clinic in a community has a daily energy demand of $$4,500\text{ Wh}$$. The solar installer specifies $$3\text{ days of autonomy}$$ to ensure that medical refrigerators and emergency lights remain powered during prolonged rainy periods. Calculate the total energy storage capacity required for the battery bank (excluding system losses).

Solution:

The given parameters are:

1. Daily Energy Demand = $$4,500\text{ Wh}$$
2. Days of Autonomy = $$3\text{ days}$$

Using the formula:

$$\text{Total Storage Energy} = 4,500\text{ Wh} \times 3$$

$$\text{Total Storage Energy} = 13,500\text{ Wh}$$

The battery bank must be designed to store a minimum of $$13,500\text{ Watt-hours (Wh)}$$ (or $$13.5\text{ kWh}$$) of usable energy to satisfy the days of autonomy.

How to Conduct a Load Audit and Determine System Parameters

This practical activity guides learners through conducting a load audit of a classroom or school laboratory, calculating its daily energy demand, selecting the system voltage, and determining the required battery storage capacity based on days of autonomy.

The required materials are:

1. A notebook and pen
2. A calculator
3. Access to electrical appliances in the school laboratory, classroom, or home

The procedure involves the following steps:

1. Identify five different electrical appliances in the room (for example, ceiling fans, fluorescent tubes, computers, or television sets).
2. Locate the specification tag on each appliance. Locate and record the power rating in Watts (W). If the tag only lists current in Amperes (A) and voltage in Volts (V), calculate the power rating using the formula: $$\text{Power (W)} = \text{Voltage (V)} \times \text{Current (A)}$$.
3. Estimate the average number of hours each appliance is switched on per day.
4. Multiply the quantity of each appliance by its power rating and daily operating hours to find the daily energy demand for that appliance type.
5. Sum the daily energy demands of all five appliances to find the total daily energy demand of the room.
6. Apply the system voltage selection guidelines to choose the appropriate nominal DC system voltage (12V, 24V, or 48V) for the room.
7. Assume the system requires 2 days of autonomy. Multiply the total daily energy demand by 2 to calculate the total storage energy required for the battery bank.
8. Record all findings and calculations clearly in the notebook.

Summary

The main points covered in this unit are:

1. A load profile is a graphical representation of power demand over a specific period, typically twenty-four hours.
2. Energy demand is the total electrical energy consumed by appliances over time, measured in Watt-hours (Wh) or Kilowatt-hours (kWh).
3. System voltage is the nominal DC operating voltage of the solar PV system components, commonly configured as 12V, 24V, or 48V.
4. Days of autonomy is the number of days a battery bank can power the connected loads without receiving charging current from the solar panels.
5. System voltage is selected based on the daily energy demand: 12V for up to 1,200 Wh, 24V for 1,200 Wh to 2,400 Wh, and 48V for demands above 2,400 Wh.
6. Total storage energy required is calculated by multiplying the daily energy demand by the specified days of autonomy.

Questions

Answer the following questions:

1. Explain the meaning of a load profile and state why it is important when designing a solar PV system.
2. Define the following terms as used in solar PV installation:
   1. Energy demand
   2. System voltage
   3. Days of autonomy
3. A household has 6 LED bulbs (15W each) used for 6 hours daily, and 3 standing fans (75W each) used for 8 hours daily. Calculate the total daily energy demand of the household.
4. Based on the daily energy demand calculated in Question 3, select the appropriate nominal system voltage (12V, 24V, or 48V) for the household.
5. If a solar PV system has a daily energy demand of 2,800 Wh and requires 2 days of autonomy, calculate the total energy storage capacity required for the battery bank.