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National Conference on Communications Analysis of a Hybrid Renewable Wind - Solar Power System for a Rural GSM/UMTS

Site. Case Study - Uganda

Paul Asiimwe Kyoma

Department of Electrical and Computer Engineering

Makerere University

Kampala, Uganda

kyomapaul@gmail.com

Edwin Mugume

School of Electrical and Electronic Engineering

University of Manchester

Manchester, United Kingdom

edwin.mugume@gmail.com

Abstract — The mobile cellular communications industry

in Uganda is one of the fastest growing sectors of the

economy. However, mobile operators are faced with high

costs of operation which hamper service provision and

reduce profit and further investment in the industry. One

of the major contributors to the total operating costs is the

electricity needed to run the base station sites and other

system components. This problem is more prominent in

rural areas, most of which are not connected to the

national electricity grid and have to run on diesel

generators all the time. For such sites, service providers

incur high operating costs due to the high cost of diesel and

associated generator maintenance costs. Operators are

always looking for solutions to reduce their operational

expenses. This paper proposes low cost and economical

configuration of a stand-alone PV/wind hybrid energy

system for a typical GSM/UMTS site in rural Uganda. The

meteorological data of solar insolation and wind speed for

a typical rural area in Uganda, and the energy

consumption of a typical rural site are studied and

simulated using the HOMER energy modeling software.

The simulations show that such a power system can

reliably run a typical rural site. The proposed system is not

only more environmentally friendly than a diesel generator

system but also more cost effective in the long run

considering the ever increasing fuel prices. This hybrid

system also reduces maintenance costs and makes it

cheaper for operators to roll out sites in rural areas.

Key words – rural, mobile communication, hybrid, operation

I. INTRODUCTION

Operators of Global System for Mobile Communications

(GSM) and Universal Mobile Telecommunications Systems

(UMTS) in many rural areas in Africa incur high operational

expenses (OPEX) due to limited extent of the national

electricity grid in such areas. In Uganda‘s case, most rural

areas are not connected to the national electricity grid and

operators are forced to run diesel generators in a 1+1

configuration. Due to high costs of diesel and the fact that two

generators are always required (one for redundancy), power

costs account for over 60% of the total OPEX per site [1].

This makes it very expensive for the network operators to

install GSM/UMTS sites. For rural areas where there is less

potential for acquiring new mobile subscribers, the average

revenue per user (RPU) is low which makes the operators to

shun such areas. Operators must therefore reduce their OPEX

in order to recoup their initial investment faster. Since power

related costs make up most of the OPEX, operators are always

looking for cheaper alternatives to run their sites.

With the stiff competition that characterises Uganda‘s mobile

communications sector, operators are seeking ways of

providing very quality service with minimal OPEX so as to

maximise their profits. Another issue is the need for operators

to become more environmentally friendly and reduce their

carbon footprint. According to Bell labs research, it is

estimated that base stations in the whole world produce

roughly 18 million metric tons of carbon dioxide annually [2].

The manufacture and use of information and communication

technology (ICT) contributes 2% of the total global carbon

emissions and it is estimated that this value will reach 3% by

the year 2020 [3], [4].

Due to the above reasons, operators, vendors, researchers and

other industry players are exploring cheaper, renewable and

energy efficient equipment not only to reduce OPEX but also

to enhance the effort towards greener communications. Studies

have shown that the radio access network (RAN) contributes

up to 57% of the total energy consumed by the network and

therefore, the biggest priority for operators is to find solutions

that can reduce the consumption of the RAN [5].

Fig. 1 shows the contributions of the different components of

the telecommunication network. A typical base station in a

rural area has a number of microwave transmission links for

backhaul to connect 900MHz and/or 1800MHz transceivers

that provide GSM coverage. In addition, some sites have a

NodeB for 3G coverage. In the radio base station (RBS), the

power amplifier consumes the highest amount of power.

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Figure 1. Electricity usage in a mobile network [3]

Uganda lies in the solar belt and therefore, it receives 2500-

3200 hours of sunshine per year and a mean solar radiation or

insolation of 5.1kWh/m2 per day on a horizontal surface [6].

Solar insolation is essentially a measure of the radiation or

solar energy received over a given area on the surface of the

earth per unit time.

According to Rodolfo and Sebbit in [7], Uganda has an average

wind speed of 3 m/s. This is higher in some areas such as the

Karamoja region, high altitude areas and on the shores of Lake

Victoria. Most base stations in rural areas are located on high

hills so as to provide a wide coverage footprint. Most of them

use tall masts to gain a high elevation over the surrounding

reflectors, scatterers and other obstacles which would

otherwise hamper the signal propagation. Thus, in general,

rural base stations are located in areas with a high wind speed.

This means that wind and solar have potential to provide

alternative energy sources that can ultimately contribute to the

total energy requirements of the network. Diesel generators can

then be used as back up especially during major maintenance

works.

The purpose of this paper is to analyze the potential of a hybrid

solar and wind generation system as a solution to provide extra

energy that can be used to run some of these components. It

will provide a mathematical analysis of a typical rural base

station and discuss qualitatively the potential energy savings,

pay back period and provide an optimum system design

suitable for rural deployment.

II. CASE STUDY

A site in Kalangala District (coordinates S0.3084, E32.2250)

has been chosen for the subsequent analysis in this paper.

Kalangala is a typical rural area in Uganda but its tourism,

palm oil and fishing industries are booming. Thus, such a place

would easily pass the initial marketing feasibility study because

of the high traffic potential it possesses. Different operators in

Uganda have stepped up their coverage in Lake Victoria and

the fishing villages in and around it so as to provide sufficient

coverage to the fishermen and tourists.

In the analysis, typical Alcatel-Lucent equipment was used for

the system design. This included a 900MHz GSM base station,

a UMTS NodeB and one microwave link. An outdoor site was

considered so that more energy efficient fans can be considered

as opposed to indoor base station sites which require air

conditioners. Using specifications given by Alcatel-Lucent, the

electrical load of the site was calculated. Using NASA data for

this region, the wind speed and solar insolation was obtained

[8]. A battery bank was considered for storing the excess

power from the solar and wind system. This was obtained from

the simulation using HOMER. Fig. 2 shows a schematic of the

hybrid power system that was designed to run the site. Each of

the power sources will be dimensioned in the next section.

Table 1 shows data was obtained from NASA for the

Kalangala area that includes the solar insolation and wind

speeds. It can be seen that the average solar insolation on a

horizontal surface is 5.1kWh/m2 per day and the average wind

speed at a height of 50m above the surface of the earth is

4.6m/s.

Figure 2: Schematic diagram of hybrid solar-wind for GSM/UMTS site with a

possible diesel generator backup.

TABLE 1: METEOROLOGICAL DATA FOR THE SITE [6]

Month Insolation (kWh/m2) Wind velocity (m/s)

Jan 5.2 4.1

Feb 5.7 4.4

Mar 5.6 4.6

Apr 5.0 4.7

May 4.7 4.9

Jun 4.8 5.2

Jul 5.0 5.0

Aug 5.2 4.8

Sept 5.4 4.8

Oct 5.0 4.5

Nov 4.7 4.2

Dec 4.9 3.8

Average 5.1 4.6

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Most of the base station equipment uses direct current but there

is equipment that uses alternating current. For example, a

socket may be provided for charging laptops which are used

during installation, commissioning and maintenance of the base

station. Thus, an inverter is normally used to supply power for

such equipment.

Table 2 shows the average hourly energy/power consumption

by each component of the site obtained from the specifications

of the equipment. Thus, the total power required to run a base

station site with this equipment is 1783 Watts.

In the following analysis, two scenarios have been identified.

In the first scenario, the required power is provided by either

solar or wind system independently. In the second scenario, a

solar-wind hybrid system is used to provide the required

power to run the site. Observations and conclusions are drawn

based on results of the analysis.

Scenario A: Stand-alone system

Average Energy requirement per day = 178324 = 42,792Wh

Wind Turbine dimensioning

The power generated by a wind system can be expressed as:

3

2

1ACP p (1)

P is the generated power available, pC is the efficiency of

the turbine, A is the area of the turbine (2rA where r is

the rotor radius), is the wind velocity and is the density

of air. According to the International Standard Atmosphere,

air has a density of 1.22 kg/m3 at sea level and a temperature

of 15oC. Assuming an efficiency of 25%, the rotor radius

required to generate power of 1783W for an average wind

velocity of 4.6m/s is obtained from equation (1) as:

mC

Pr

p

8.6*2

3

(2)

TABLE 1. AVERAGE POWER REQUIRMENTS

Equipment Power (W) Number Total (W)

Compact BTS

900 Outdoor

1283 1 1283

UMTS NodeB 300 1 300

1 Socket (for

laptop)

100 1 100

Microwave

(IDU+ODU)

100 1 100

Total 1783

Solar Photovoltaic Array dimensioning

In the case of a solar system, it will be dimensioned to produce

130% of the power which will cover about 25% of the losses.

Therefore, the required solar panel power rating can be

expressed as: )(*3.1

Wattsinsolationsolar

LoadP

(3)

This design considers the worst case scenario to deliver a

power system with the highest reliability. Therefore, the

lowest solar insolation value of 4.7kWh/m2 (corresponding to

May and November) was used in equation (3). For a load of

42,792Wh, the required solar panel power rating is:

P = 11836Wp

Scenario B: Hybrid System

An independent solar system would have the weakness that

solar power can only be harvested during the day. A hybrid

system combines both the solar and wind systems to make one

power source that optimizes the cost of powering the base

station. Dimensioning such a hybrid system can be based on

any of four criteria:

a) Splitting the cost of the hybrid system 50-50.

b) Splitting the power generated by both systems 50-50.

c) Optimizing the total cost for the combined hybrid system.

d) Fixing power of one system and dimensioning the other.

Since the major reason why operators would consider a hybrid

renewable power system is to reduce their OPEX, then the

third criteria is preferred and has been chosen for this analysis.

However, there are other constraints that must be considered

during the design of such a system. These include:

The space available for the site might dictate the size of

the solar array and the size of the wind turbine.

The turbine can be accommodated on the same tower as

the radio antennas. However, this has to be taken into

account during the civil design of the tower as the turbine

can add significant weight. Otherwise, a separate tower

may be built for the turbine although this increases the

total cost of the site including site lease costs.

A number of cost optimization methods exist. These are

probabilistic, iterative or graphical [9]. For this paper

however, HOMER (Hybrid Optimization Model for Electrical

Renewables) simulation tool was used to optimize the cost of

the system for a typical rural site.

According to European Wind Energy Association (EWEA),

the average cost per kW of wind power ranges between $1,000

to $1,265 while the average cost per kW of solar energy

ranges between $3,300-$4,400 [10], [11].

From the simulation results, the cheapest system will consist

of a 5kW solar array, two generic 10kW wind turbines at a

hub height of 40m and 48 160Ah Narada batteries. Such a

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system would cost $49,000 (1 USD = 2,600 Uganda Shillings

at the time of writing this paper).

Fig. 3 shows the possible combinations of the PV photovoltaic

and wind systems that can be used for optimal system

operation. The highlighted one in the figure shows the most

optimal configuration considering the excess power that will

be produced. The system summary is shown in Tables 3 and 4.

Fig. 4 shows the percentage contribution of the different

power sources towards the total power output of the system. It

can be seen that most of the power contributed by the wind

sub-system. This is the preferred scenario because the cost per

kWh of wind is cheaper than that of a solar photovoltaic

system.

III. OBSERVATIONS

From the calculations and simulations, it was observable that:

For enough wind power to be harnessed to supplement the

power generated from solar, big wind turbines have to be

used. This will significantly increase the initial investment

cost of the project because of the huge turbine and a tower

on which it has to be installed. It will also increase the

amount of space that the operator needs for the site which

increases cost of site lease. For this reason, two small wind

turbines are proposed instead of one.

Figure 3. Most optimal hybrid system combinations

TABLE 3. SYSTEM COMPONENTS

PV Array 5 kWp

Wind turbines 2 Generic 10kW at a hub height of 40m

Battery 48 Narada (each rated 160Ah).

TABLE 4. ELECTRICAL POWER DISTRIBUTION

Component Production (kWh/yr) Fraction

PV array 7,427 19%

Wind turbines 31,369 81%

Total 38,795 100%

Figure 4. Monthly average electric production

If only solar energy is to be used, the total size of the solar

array needed to generate significant solar power becomes

very large. Thus, the solar array will require a very large

area for its deployment. In addition to the high cost per

kWh of a solar system, the costs associated with site lease

will also increase.

The wind and solar hybrid system can produce more

power than is necessary to run the system. However, there

are also periods when the generated power may be less

than the load. This problem is countered by having bigger

battery storage to store any extra energy and feed it back

into the system later. This problem can also be countered

by having a standby generator that provides any power

shortfalls when required. Such a generator should be able

to supply the 1783W required by the site. A 2kW

generator should be sufficient for such a purpose. It is

recommended that such a generator be a mobile diesel

generator as it might be redundant of the time.

A mobile standby diesel generator may be required in

cases where major maintenance work on the hybrid system

is due to take place or when the system breaks down. This

will reduce the downtime of the site and enhance network

availability further.

Such a hybrid system also has significant advantages over

current sources of power. Once installed, it reduces the fuel

costs and CO2 emissions by 100%. This system also does not

require a high maintenance effort and maintenance costs can be

reduced significantly by over 80%. Considering that such a

system has a life span of over 10 years, it leaves ample time for

operators to regain their initial investment and then enjoy

significant profits. It therefore makes good business sense for

operators to consider it for most of their rural sites which are

very difficult to connect to the national grid and for which they

incur high OPEX with little return on investment (ROI).

IV. CONCLUSION

The analysis presented in this paper shows that there is enough

potential to exploit solar and wind energy to provide power in

rural areas in Uganda. In the context of cellular communication

systems, an entire stand-alone base station can be supported on

power generated by a solar-wind hybrid system at all times.

However, a battery bank would be needed for redundancy

purposes and will be charged by excess power generated by the

hybrid system. Such a power source would significantly reduce

the OPEX of operators, increase their desire to invest in rural

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areas and hence enhance penetration of telecommunication

services in rural areas.

As governments start implementing climate change policies

and as operators continue to seek more energy efficient power

solutions, Uganda should not be left behind. The world is

facing an energy crisis and Uganda is not immune. Fuel prices

are very unstable but the general trend is that the prices are

increasing. This reduces the profit margins of operators in

Uganda‘s competitive mobile telecommunications market.

Implementing a hybrid solar and wind system to run their base

stations in rural areas would go a long away in reducing their

costs, improving the potential for further investment in the

sector and crucially, it would reduce their carbon footprint.

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