Mechanical Engineering Electronic & Electrical Engineering

Group Activity XX50180

Micro generation scheme- part 5:

Energy consumption analysis of domestic appliances

Student: Wei WEI

Student number: 106477885

MSc Programme: Electrical power systems

ABSTRACT

Solar energy is regarded as one of the very promising renewable energies in the future developing. Not only does the industry use solar power, but also are the domestic energy systems now trying to integrate their existing system with solar energy. However, solar energy brings problems to the existing energy systems. The safety operation and stability of the existing energy systems are the major aspects.

In this report, with analysis of household types and appliances, load profile which is related to the demand response and energy storage will be modelled for a certain type of household and protection systems for micro generation system will be carried out.

Key word: load profile, demand response, energy storage, electrical appliances and protection system.

Chapter 1 Introduction

1.1 Background and motivation

Energy consumption now is a heated topic in many fields in people’s daily lives. In UK, the government sets the target of reducing 80% of greenhouse gas emission based on 1990’s level by 2050. Domestic renewable energy (micro generation) is an important field of helping to achieve the target.

However, with the increasing load, the existing network has insufficient capability to integrate the renewable energy systems, so different ways of energy consumption and protection methods are involved in domestic renewable energy — demand response (DR) and protection schemes.

With the load profile data, demand response and energy storage can help to make a new energy consumption scheme. DR can increase, decrease or shift the energy requirement according to different information, such as electricity price, peak demand, etc. Normally, most DRs are for large energy users, such as industry and commercial, with less focus on domestic users.

The micro generation will connect to different terminals, namely AC (the grid) and DC (appliances), so different protection schemes should be used.

1.2 Objectives

This report will carry out:

1) The load profile of one typical household based on domestic appliances and operation times

2) Demand response to the household

3) Current methods of demand response

4) Protection schemes for bus-bars.

Chapter 2 Appliances and Load profile

2.1 Domestic appliances and households information

2.1.1 General idea of classifications

Domestic appliances indicate the electrical/electronic/mechanical equipment that helps to finish household functions. Home appliance can be classified into three species:

White goods represent the major appliances, such as air conditioner, dishwasher, tumble dryer, freezer, refrigerator, stove, water heater, washing machine, microwave oven and induction cookers. Because the major goods were typically painted or enamelled white, so they are called white goods.

Brown goods represent the small appliances, such as TV set, CD, DVD player, camcorders, still cameras, clocks, video game consoles, telephones and answering machines. The small appliances were made of wood, so the name has stuck. [1]

Shiny goods represent the customer electronics which are the electronic equipment intended for everyday use. [2]

2.1.2 Ownership of domestic appliances

According to a research [3] “Energy Consumption in UK Households: Impact of Domestic Electrical Appliances” carried out by Iman Mansouri, Marcus Newborough & Douglas Probert, Department of Applied Energy, Cranfield University, Bedford, UK, the ownership of appliances is shown below:

Fig. 1 ownership level of domestic appliances [5]

In this research, it also pointed out that the electricity used to support space and water heating is very low as shown below:

Table 1 general percentage share of different types of house heating system in 2000 [3]

To simplify the load profile analysis process, the assumptions are made to analyse the load profile:

The electrical boilers will not be considered, as they possess very little amount.

This report will focus on the major electricity assumption appliances

The chosen type of household will have the same appliances.

Some low level demand and low usage appliances will not be considered in this report, such as small electronic appliances.

One household only has one of each appliance.

The load profile is on weekdays for full-time worker.

2.1.3 Household sizes and types

Load profile varies with three aspects of household, number of person, life style and appliances respectively. The following table charts show the basic household information of the combination of different household types.

Table 2 percentage share of different numbers of people in a household in UK [5]

Table 3 percentage share of household types in UK [5]

According to the research information, there will be 5 scenarios of energy consumption:

Table 4 different energy consumption scenarios in UK [4] [5]

2.1.4 Energy consumptions

Load profile is a comprehensive description of energy usage profile. In 2000, a research was carried out and gave a clear data of energy level of different appliances which are shown below:

Table 5 percentage share of energy consumption of different household appliances [5]

For office-worker this type of household, this report will focus on certain appliances that have been used in a typical office-workers’ house which are electric hob, electric oven, microwave, refrigerator, TV, washing machine, tumble dryer, dishwasher and kettle. For other unpredictable consumption of appliances (such as PV) which depend on personal lifestyle and low energy level appliances (such as lighting) will not be shown in the report. Considering the electricity consumption will be different according to the number of people in one type of household, this report will concentrate on the majority type — two adult without children, according to the data from table 2 and table 3.

2.2 Load profile

2.2.1 Definition of load profile and average residential load profile

Load profile indicates the electricity usage of the electrical loads during a certain period of time, so load profile always varies with different types of household. Load profile can be used as a prediction of the electricity consumption which is related with the efficiency and transmission of electricity distribution network. Load profile typically describes the electricity use in minutes, hours or day.

Fig. 2 average residential load profile (KWh) [15]

This is an average residential load profile. The peaks are between the hours of 6PM and 9PM during the day when people get back home and start to watch TV and cook, etc. The electricity usage starts to drop from 8 pm and reaches its lowest point at 3 am in the morning.

The load profile always has an influence on the electricity price. When the time reaches the peak demand time, in order to meet the demand, the expensive generator will get on line and generate electricity which means that the electricity price will increase.

In summary, if the customer manages the electricity consumption during the day, the electricity bill will decrease and help to decrease the line losses during the peak time.

2.2.2 Load profile of two-office-worker household type

The table below shows the major appliances’ average electricity consumption and average operation periods of two-adult-without-children type during a day.

Table 6 electricity consumption and operation periods of major appliances in one day [4] [5]

According to the data of electricity consumption and operation periods from the research (table 6), load profile of two office workers is shown below.

Table 7 load profile of two office-workers in one day (unit in KWh)

The time interval is based on per hour and data is allocated evenly when the appliances are operating. For some certain appliances, the electricity consumptions are lower in the morning and more in the evening because breakfast is normally simpler than dinner and it is possible that people cook the next day lunch in the evening.

Using the data in table 7, figures of electricity load profile from three different aspects are:

Fig. 3 Electricity consumption of different appliances in one day (KWh)

Fig. 4 Operation periods profile of different appliances in one day (KWh)

As seen on the figure, electric hob, and dishwasher use more electricity in one day and the refrigerator operates 24/7.

Fig. 5 Electricity consumption in one day for two office-workers (KWh)

Figure 4 shows the load profile of major appliance in this type of household. The peak electricity consumptions are at in the morning starting from 6am to 9am and in the evening from 6pm to 11pm. The energy consumption is much higher in the evening after work.

In 2012, the weekdays, weekends and holiday are shown below:

Table 8 days in 2012

As an office worker, the life style is very regular, so in the most of time the electricity consumptions are the workdays. There will be difference between holidays and weekdays, but considering the little amount of the days, it can be considered as weekdays.

In table 7, the total electric consumption is 7.33 KWh, so the total electricity consumption in 2012 will be

7.33 KWh ×365 days = 2675.45 KWh / year

Chapter 3 Demand response methods

3.1 Definition of demand response

Federal energy regulation commission gives a definition of “Changes in electric usage by end-use customers from their normal consumption patterns in response to changes in the price of electricity over time, or to incentive payments designed to induce lower electricity use at times of high wholesale market prices or when system reliability is jeopardized.” [6]

3.1.1 Demand response for household type — office-worker

For the office workers, because they have very regular time of operating all the appliances, it is impossible to change the cooking time. However, if the customers can shift the operation times of tumble dryer, washing machine and dishwasher, the large electricity consumption appliances in the household, to the low electricity price periods or use the energy storage, the electricity bill will be reduced as well as help to reduce the electricity generation during normal peak demand time.

In the next two parts, 3.3 and 3.4, two main technologies related to demand response will be described.

3.2 Load control switch

Load control switch is the equipment that applied on the large electricity consumption home appliances. [7] This kind switch helps to decide whether turn off or on the appliances according to the electricity price and energy stored.

According to the definition in Wikipedia, there are two types of switches: [7]

Switch with the communication module can receive signal from the power company and from installed energy back up system (energy storage). If the electricity price is high or there is not sufficient energy stored in the energy storage system, then the appliance will not operate. However, if the price is low or there is enough stored energy, it will automatically start the appliance.

Stand-alone switch doesn’t have and communication module. They are applied individually on the appliances monitoring the electricity consumption. Customers choose when to use the electricity when to cut off.

With the load control switch, it helps to reduce the load demand during the busy time and helps to keep the power system stable.

3.2.1 Changing frequency of the grid

The frequency of the grid is changing according to the load condition. The figure below shows the real time frequency data (Fig 6 01:30 pm 18/4/2013; Fig. 7 06:00 pm 18/4/2013; Fig.8 09:00 pm 18/4/2013)

Fig. 6 real time frequency data [8]

Fig. 7 real time frequency data [8]

Fig. 8 real time frequency data [8]

The graph above shows the real time frequency data to a 15 second resolution over the last 60 minutes. [8] Normally the grid frequency is around 50 Hz and it will be kept stable, but if the load demand has a massive change, the change will affect the system frequency. Therefore, the frequency is constantly changing because of different load demand. The grid frequency has a characteristic called drooping characteristic.

Fig. 9 drooping characteristic in per unit (P.U.) [9]

As can be seen from the figure, when the power output increase (increasing load demand), the system’s frequency will drop, vice versa. R is the slope of the line; is the changing frequency of the grid; is the change of power output.

3.2.3 Detecting frequency change (electricity price)

By detecting the imbalance of the frequency in the real time, it is possible to control the operation of the appliances. If the grid runs at low frequency (down to 49.00Hz say), it will switch off from the grid instead of electric cut off. And then, the appliances can use the energy stored in the energy storage system or use energy from solar generation directly. [10]

The following figure shows an example of a load control switch with the tumble dryer. [11]

Fig. 10 example of load control switch with tumble dryer

3.3 Energy storage with load control switch

Energy storage always comes with the renewable energy. With the energy storage battery, customers can choose to use the stored energy instead of the grid during the peak demand time in the day time or when they suffer an electric black-out. The battery type and capacity will be discussed in part 3, group 9.

Battery has its depth of discharge which indicates that the battery need to be charged when reaches its certain “depth”. If below the level, the battery is permanent damaged or shorter life. The storage battery bank needs to supply the power between charge cycles. Making sure the battery capacity is about 8-10 hours is a perfect minimum. [Part 3, group 9] [12]

Chapter 4 Protection

The micro generation system is connecting to both terminals: 1. User-end, to supply appliances at home. 2. The grid, to earn money from feed-in tariff. It is important to have protection system in both ends.

4.1 Bus-bar protection

Every appliance will connect to the bus, so the protection of bus is very important.

The protection scheme is current differential protection. The working principle is based on the Kirchhoff’s current law: the sum of currents flow into the node should be zero.

Fig. 10 current differential protection [13]

Under normal condition, there is no current difference of two points on the bus-bar, so the CT (current transformer) will not sense any difference and there will be no current flows into the relay.

However, if there is a fault occurred, the CTs will sense the current different and the relay will be tripped. The circuit breaker will work to protect the bus-bar and the appliances. [13]

Current differential protection is always used for unit protection, such as feeder circuits, bus-bars, transformers and generators. The working scenarios are:

When current into the plant equals current out, Ia=Ib: [14]

Relay restrains—no operation

When fault exists on the bus-bar, Ia≠Ib:

If If’ >I setting, relay operates.

Chapter 5 Conclusion and future work

Load profile and demand response are the very important aspects in the energy saving. Both of them bring benefit to the customers and the grid. With the help of load profile and demand response, the power grid can be operated stably and the customers will have reliable supply and earn money.

The load profile is depending on the household types. This report focuses on the two office-workers. With the reasonable assumptions, the load profile is obvious under normal conditions.

The demand response helps to shift the occupation time and electricity supply of the appliances during the whole day from which the electricity bill will be reduced as well as make money from generated energy.

The protection scheme helps to make sure that every part of the micro generation system will work properly and protects both the appliance and storage system.

However, there are many assumptions in this report for simplifying the analysis process. In the future work, it is necessary to develop a method to give accurate load profile data of different households and able to work on other types of household.

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