High level technical design and economic assessment of renewable energy solutions for radio base stations

TABLE OF CONTENTS

1 Introduction
1.1 About this document
1.2 About the author
1.3 Issue definition
1.4 Scope
1.4.1 In scope
1.4.2 Out of scope

2 Mobile operators
2.1 Role and motivation
2.2 Technical context
2.3 Technical characteristics

3 How to get RES@RBS
3.1 Network perspective: prerequisites (macro view)
3.2 Site perspective: planning steps (micro view)

4 Selected technical recommendations
4.1 General
4.2 Technical design
4.3 Solar
4.4 Wind
4.5 Batteries
4.6 Charger, controller
4.7 Diesel generator
4.7.1 Accessories (cabling)

5 High level RES assessment (RAFORS)
5.1 Purpose
5.2 Abstract
5.3 Simplifications / restrictions
5.4 How to work with RAFORS
5.4.1 General
5.4.2 Input data
5.4.3 Output
5.5 Calculation
5.5.1 Power
5.5.2 PV
5.5.3 Battery
5.5.4 Wind
5.5.5 Diesel generator
5.5.6 Economic aspects
5.6 Way forward

6 Definitions
6.1 Glossary
6.2 Abbreviations
6.3 Abbreviations (formulas)
6.4 Figures
6.5 Tables

7 Appendix
7.1 Example of a ‘Site questionnaire to plan RES based radio stations’
7.2 Example of a 3-phase design
7.3 Example of an RES@RBS assessment with RAFORS
7.4 Suppliers (examples)
7.5 Symbols (electrical)

1 Introduction

1.1 About this document

This document describes a high level technical design for and the economic assessment of renewable energy solutions for radio base stations (RBS). It proposes a way forward in the development of 'hybrid' solutions with renewable energy sources (RES) for off-grid RBS sites.

The main purpose of this document is to provide initial practical, technical and economic guidance. It is accompanied by an easy-to-use EXCEL-based tool (RAFORS – Renewable energy Assessment FOr Radio Sites) for the assessment of the net present costs (NPC) of these solutions.

The following sections outline the specific requirements of mobile operators and provide specific technical guidance for the selection and implementation of the elements of a hybrid system.

This information will enable mobile operators without any detailed technical and commercial knowledge to develop initial ideas on the design of such a system and provide them with a common basis for further discussion, evaluation and benchmarking.

References to external documents in the glossary are in square brackets [ ] and cross links to specific sections of this document are indicated as follows: >> section_name.

Note: this document is part of my final examination in the renewable energy study module at the University of Kassel. The precise definition of the term paper can be found in >> Issue definition

1.2 About the author

Comments, remarks and questions about this document are welcome! Just send an e-mail to

Heinz.rueterbusch@gmx.de

The author is working in Technology Network Department of an international mobile operator on energy and Infrastructure solutions to support the operating companies.

He is a member of the Environmental Engineering (EE) Group within the European Telecommunications Standards Institute (ETSI).

1.3 Issue definition

Subject: <High level technical design and economic assessment of renewable energy solutions for radio base stations>

The aim of this paper is to describe and discuss the different aspects of the technical design and economic assessment of renewable energy solutions for radio base stations. The following points should be considered:

A. Load profiles, user requirements, resources, typical technical concepts for radio base stations.

>> Technical characteristics

B. Renewable energy-oriented technical design. Possible concepts, adaptation of different energy sources, integration of the components (PV, wind generator, diesel, batteries, chargers/inverters, management/control, accessories) >>How to get RES@ and >> Technical design

C. Economic assessment and comparison of different technical solutions >> High level RES assessment (RAFORS)

D. Design of a “Site questionnaire to plan RES based radio stations” >> Example of a ‘Site questionnaire to plan RES based radio stations’

E. Presentation of an example (including schematic diagram and nominal characteristics of the components) >> Example of an RES@RBS assessment with RAFORS

Note: >> indicates the section in which the aspect is covered.

1.4 Scope

1.4.1 In scope

- Off-grid RBS sites
- Renewable energy
- Technical aspects
- Economic assessment

1.4.2 Out of scope

- Detailed site-specific planning
- Switching centres
- Specific suppliers

2 Mobile operators

2.1 Role and motivation

Two of the key challenges facing mankind today are protecting the environment and combating climate change. Mobile operators are part of the information and communication technologies (ICT) sector and, as such, they play a key role since they are both part of the solution and part of the cause.

illustration not visible in this excerpt

Figure 1: Example of how ICTs are contributing to global warming [1]

The illustration shows how telecommunication services can help to prevent global warming by providing an observing system.

ICTs consume power, radiate heat and emit carbon dioxide due to the operation of the telecom network >> Technical context. The objective is to reduce carbon dioxide emissions because these emissions are what cause global warming. A reduction of carbon dioxide emissions can be achieved principally by

- improvements in energy efficiency and
- increased use of renewable energy sources (RES).

The challenge is to achieve energy-efficient and sustainable mobile communications through network and site optimisation and the use of renewable energy sources.

The use of RES will also help to reduce future operating expenditure (OPEX) on energy. However, the full net present costs (NPC) and financial expectations also have to be taken into account when considering alternatives.

2.2 Technical context

The technical devices that are necessary for the operation of a mobile-to-mobile connection can be logically assigned to a ‘core’ network and an ‘access’ network. The access network contains the radio base stations (RBS), which are also called sites or base transceiver stations (BTS).

illustration not visible in this excerpt

Figure 2: How a mobile network works, similar to [8]

The exchange switch is part of the core network. Around 80% of expended energy and the related carbon dioxide emissions are accounted for by the access network due to the number of RBS sites. Today’s European networks operate two technologies concurrently: 2G (GSM) and 3G (UMTS).

The RBS equipment mainly operates on direct current power (DC), with the exception of the conventional air-conditioning (AirCo or cooling) systems which cool the equipment and batteries >> Batteries. The following table shows the items of RBS equipment and their typical power consumption.

illustration not visible in this excerpt

Table 1: Power consumption at a co-located radio site

RBS sites mainly require DC power and the radio equipment has the highest power consumption.

Due to the fact that that RBS operate 24h/365d and the impact on traffic variation is less than 20%, an almost constant load throughout the year is assumed.

Unlike radio equipment, cooling systems are not operated 24h/365d. However, as a result of outside temperature fluctuation, it is practical to evaluate load requirements in the summer and winter seasons and to take the worst case (highest value). Some hybrid planning software such as HOMER (>> Site perspective: planning steps) can define daily consumption profiles for different DC sources.

Since it is assumed that mobile operators will be able to reduce the use of conventional cooling systems, the RAFORS model only takes DC load into account. This is possible because the ETSI EN 300 019 [1] environmental standard allows the environment in which the equipment is installed to vary. For example, in class 3.1 room temperature can vary from 5°C to 40°C and in class 3.2 (partly temperature controlled) room temperature can vary from -5 to +45°C [3].

Technological advancements which boost the efficiency of radio equipment will lead to lower power consumption in future, and the typical power consumption of an RBS is expected to decline to around 2 - 3kW.

2.3 Technical characteristics

The following section outlines the key technical characteristics of RES applications which differ from island applications such as small villages:

- Mainly DC loads, operating 24h/365d
- Most common DC voltage is -48V
- Relatively small loads (less than 5KW on average, typically 2 - 3 KW)
- Relatively stable and predictable long-term loads (through the use of new technologies)
- High numbers of radio sites (e.g. Vodafone has around 85,000 sites [8] worldwide)
- Space restrictions, especially in urban areas

Several technical solutions (e.g. grid, diesel engine etc.) are available to power radio sites, so it is important that an RES solution has a comparable total cost of ownership (TCO), expressed in terms of net present costs (NPC).

These circumstances make the application or evaluation of renewable energy for pure off-grid sites more feasible since off-grid sites

- have less space restrictions (e.g. for photovoltaic (PV) modules)
- enable clear benchmarking with costs of diesel engines
- have long maintenance intervals (no fuel logistics)

Aspects such as the solar panel theft and the incidence of the renewable energy sources have to be considered on a case by case basis.

Maintenance and site visits also have to be made to off-grid sites at least once a year.

3 How to get RES@RBS

3.1 Network perspective: prerequisites (macro view)

Energy optimisation in mobile communications is a step-by-step process, starting with an energy efficient network design followed by an energy efficient RBS site design [3].

An energy efficient site and network design (macro view) - also in terms of legacy elements - can be achieved in the long term in the following order by

1. Designing to real needs
2. Optimising the existing infrastructure or radio equipment
3. Introducing more efficient technology
4. Considering RES as a potential power source (site specific, micro view)

Guidance on the first three bullet points already exists in the form of the ETSI's Technical Report on 'The reduction of energy consumption in telecommunications equipment and related infrastructure.' [3}. This document is an accumulation of ideas from operators and manufacturers on the methods to increase the energy efficiency of telecommunication systems in order to reduce operational energy requirements.

For example, the introduction of higher temperature set points enables the replacement of conventional 3-phase cooling units with free cooling ventilation units which consume less power and operate on DC power. Please remember, however, that elements of the design to real needs phase may have an impact on the entire network. It could take quite some time to complete since existing operations with related organisational processes are affected.

When site optimisation has taken place, RES can be considered from a site-specific perspective.

3.2 Site perspective: planning steps (micro view)

The following illustration shows one possible approach from the assessment phase to the site-specific implementation of a renewable energy solution. The level of detail increases progressively over time from left to right.

illustration not visible in this excerpt

Figure 3: The ‘RES@RBS’ planning concept

Off-grid systems, particularly hybrid systems, are characterised by a high degree of complexity at the dimensioning stage. That's why software simulation tools are extremely useful. [13]. The term ‘hybrid’ system in this document refers to the elements covered in >> Selected technical recommendations

One possible initial assessment tool is discussed in >> High level RES assessment (RAFORS). Several tools for the other introduction phases are also available. Planning software for full hybrid solutions can be placed in the categories of

- dimensioning programs (Dim), which calculate the system dimensions on the basis of input data (load and climate data and system components), and
- simulation programs (Sim), which use the input data (load and climate data, system components and configuration) to simulate the behaviour of the system over a given period.

A good summary is provided by [13].

The following table shows the software tools used in the planning phase.

illustration not visible in this excerpt

Table 2: Example of planning software tools

One of the key decision criteria for users selecting hybrid tools is the kind of calculations that the tools can make: economic calculations (HOMER), general dimensioning (RETScreen) or a detailed technical configuration. Alternatively, the planning can also be performed manually in the following logical order:

1. Assessment of resources (wind, solar)
2. Dimensioning of the PV generator, wind generator (power and type)
3. Battery dimensioning
4. Selection of charger and inverter

Many of the software tools include wind or solar databases, so any site questionnaire should include the geographical location of the site >> Example of a ‘Site questionnaire to plan RES based radio stations’

The first consideration should be solar energy (photovoltaic, PV), because

- PV energy yield is easy to predict (solar radiation)
- PV systems are simple to install and maintain and therefore
- PV systems reduce operational costs to the minimum.

The following sections provide an overview of the components in a hybrid system and the prediction of natural resources.

4 Selected technical recommendations

4.1 General

The following section provides guidance on the selection and design of the technical elements. This document does not recommend a specific vendor. However, for practical reasons a number of suppliers and products are presented in the annex >> Suppliers as an example.

Initial pointers on the manual assessment of solar and wind resources are provided at the end of the wind and solar section.

illustration not visible in this excerpt

4.2 Technical design

Although the load requirements are mainly DC, the air-conditioning system may require some AC. Diesel generators are also used as a back up or main power source at most sites. As a result, a ‘mixed’ design has to be applied. The diagram below shows the two possible designs.

illustration not visible in this excerpt

Figure 4: Recommended technical designs: DC only and mixed (AC and DC)

The power generators are on the left hand side of the diagrams and the BTS load is on the right hand side in the shadowed boxes. A key to the symbols is provided in the annex > Symbols.

The bi-directional battery charger is a bottleneck in the ‘mixed’ design on the right hand side. However, this might be an acceptable drawback since relatively stable or even decreasing load demand is expected in future >> Technical characteristics.

[...]