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Thermal Solutions to Reduce Electricity Costs

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Executive summary

This report investigates and analyses the thermal solutions to reduce the electricity cost for senior school at St Aidan’s Anglican Girls’ School. Approaches of analysis include calculating energy loss, thermal current and comparing current costs and revised costs, as well as the long-term (each term and annual) prediction for power consumption. The annual electricity cost is predicted to be $23064.56. All calculations can be found in the Appendix. Results from calculated data show that 29.09% of the power consumption can be reduced by replacing 36 W lights from 18 W tube light bulbs ($4.90 for two tubes, $44.10 overall) and installing the Hampton thermal curtains ($276 in total) with pelmet boxes ($183) of five years life expectancy. Hence, they are suggested to renovate every five years to maintain the efficiency of thermal curtains. Although these installations will need $503.10 initially, the payback will begin from the 7th month.

This report evaluates that the current setting of the classroom in senior school can be altered to protect the environment and minimise electricity costs. Therefore, there are some recommendations:

  • Install the Hampton thermal curtains with pelmet boxes
  • Thermal curtains should be shut all the time in order to achieve the revised cost
  • Small technology devices (e.g. projector, printer, iron and hot glue guns) usage should be minimised to reduce power usage (e.g. turn them off instead of stand-by when not using)

There are some weaknesses of the report:

  • Thermal current of furniture in the room is not considered.
  • Monthly temperature data are not exact results, they are based on past average temperature data.
  • Number of classrooms are an approximation instead of counting due to variation in classroom setting (e.g. one air-conditioner, more or less lights, fridge)

Key assumptions

  • The power is assumed to be used only on schooldays.
  • The energy loss from the recorded data (the energy required to cool down) is assumed to be the same every day.
  • The room is assumed to be a closed system, so the thermal current of windows is equivalent to the power usage of both air-conditionings.

Power Usage Analysis

Introduction

Thermal (heat) energy always tends to transfer from one hot space spreading to colder surroundings space by conduction, convection and radiation (Cambridge University Press, 2014) and its flow is affected by the temperature difference. Thermal current () is the rate at which heat energy passes through a material, which is calculated as . The thermal conductivity (k) is a measure of how well the material transfers heat; lower values of conductivity indicate better insulating material. Thermal insulators such as thermal curtains and double glazing windows are commonly used in homes or buildings to maintain constant room temperature by prevention of convection current and minimise energy usage. This report will investigate, analyse and recommend thermal solutions for the school to reduce electricity usage and environmental benefits.

Investigation and discussion

Note: All data refers to Appendix – Calculation

The data was collected by using Sparkvue temperature sensors connected to a computer. The first 30 minutes of the data was disregarded due to the sensor being exposed to direct sunlight, leading to the extremely high temperature (especially the peak at 3:19pm at 47.4°C) at the beginning of the data (Graph 1). Through analysis and calculations of the data, the internal temperature change had a positive gradient (between 5:04pm and 6:04pm) which indicated the room was gaining energy as the air-conditioning turned off (347.08 W).

Current power usage

During schooldays, 18 lights were used 1 hour 43 minutes per day and two air-conditionings were used 10 hours daily on average. The lights were found to consume a relatively low amount of energy of 1.11 kWh daily. Since the room was assumed to be a closed system, theoretically, the thermal current of the windows (137.14 kWh) should be equivalent to the power usage of air-conditioners. However, due to the information written on the air-conditioning was its upper most power usage, they would not be equal. In fact, the calculated consumption (Table 2) was less than the given data, which indicated the difference in power usage could be attributed to dissipation through the walls, floors and doors openings. Moreover, the total consumption (139.52 kWh) included the power consumption of lights and technology devices in the room to achieve higher accuracy.

Thermal solutions

Thermal solutions are reasonable and beneficial to diminish power consumption. The 36 W light bulbs can be replaced by 18 W lights, which each consume 0.56 kWh daily. Furthermore, thermal curtains are the most common thermal products, because more internal thermal convection current will be blocked, especially when the curtains are extended so that it reaches the floor. This would prevent the convection current and reduce energy flow into the room (Figure 9). Double and triple glazing of windows are commonly used as well, due to vacuum between glass panes eliminating losses by conduction and convection; however, installation is expensive, so thermal curtains are the most economic thermodynamic barriers to use in order to reduce the energy usage.

Curtain with pelmet and no gaps Figure 9 – No gap (Rogers A, 2011)

From researching thermal curtains, the Hampton thermal curtain is thermal coated which reduces a maximum of 25% energy loss (Energy.GOV, 2012). Only the front windows have contact with direct sunlight; others are all under shelter, so thermal curtains will only cover these. Hence, the Hampton thermal curtain is suggested to be installed for energy reduction, costing $276 in total. Pelmet boxes cost $183 overall, because the side of the front windows have a length of 6.02 m each, it would be reasonable to get two 6.10 m pelmet box on each side(some space for the curtain rod). The life expectancy of thermal curtains is usually five years; it is reasonable for the school to renovate new curtains every five years. It can be seen from Graph 2 that the costs maintain the same and increases by another full cost of $503.1 after renovating at the end of each five year cycle. Furthermore, the power usage with thermal solutions is calculated to be 97.25 kWh, having a 29.09% difference compared with the current settings.

CBA – Cost Benefit Analysis

The CBA graph (Graph 3) shows the current cost model and revised cost model accumulating over time. The break-even point occurs in the 7th month and when the current cost is approximately equivalent to the revised cost at $1661.8 and $1654.3, the current cost will exceed the revised cost. According to the CPI in Queensland has been fluctuating from 2007 to 2012 (Figure 10), which may continue changing in the next few years. By considering a long-term situation, Graph 4 illustrates an example of revised cost will increase every five years due to renovation of curtains, whilst maintaining a smaller rate of power consumption than the current cost. The power consumption for each term is calculated (Table 3-6) and used to investigate the annual revised cost of $23064.56 and 28.98% of electricity cost will be reduced from current cost in the senior school. If the electricity price increases 5% for both peak and off-peak hours, the trend would stay the same as the current cost has a higher gradient than the revised cost (Graph 4) and the fluctuation has an impact on the overall costs.

(Australian Government, 2015)

Figure 10 – CPI Annual Change Brisbane

Recommendations

The ultimate goal of diminishing the energy usage is to protect our environment and minimise energy consumption.

Therefore:

  • Thermal curtains with pelmet boxes and light bulbs should be installed.
  • Curtains should be shut all the time in order to achieve the revised cost.
  • It would be the most economical way if all small technology devices are not used at all (e.g. projector, printer, iron and hot glue guns).

In conclusion, after installing the thermal solutions it is predicted to spend $23064.56 annually in the senior school, and results in saving approximately $13072 on power usage when all assumptions are valid, which will therefore reduce environment impact.


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