Demand Response in Smart Grid – A Systematic Mapping Study

Growing demand for electricity and the innovations in smart grid technologies is providing new opportunities for intelligent electrical demand loads control in the form of Demand Response (DR). In this paper, we present a comprehensive review of demand response by conducting a systematic mapping of the field to evaluate the current research trends, knowledge gaps, and the scope of future research of DR in the smart grid applications. Bibliometric analysis has been done to map the research themes under the demand response domain by analyzing the co-relationships in the metadata originated from the SCOPUS database. Thorough content analysis has also been performed, and six emerging research themes have been identified along with fundamental research questions commonly addressed to devise future research directions.


INTRODUCTION
Recently, the electricity grid has been undergoing rapid change because of energy conservation as well as environmental sustainability. Ageing electrical power grid infrastructure and demand for improving operational efficiency call for a reliable electrical power grid. However owing to the increasing proliferation of renewable energy sources, the electrical power grid and the power grid operators are facing operational challenges to meet operational reliability [1]. Furthermore, governments are also imposing grid operators with strict policies and regulations to cut carbon emissions as well as enforcing massive integration of renewable energy in the generation mix. Renewable energy sources (RESs) are highly intermittent, which can lead to additional ramping of generators in the grid hence compromising the grid stability.
Demand Response (DR) refers to shift in the usage of electricity by energy users from their regular use habits in response to increase in electricity rates over time or opportunities to limit the use of electricity at periods of elevated wholesale market costs or when the efficiency of the grid is undermined [2]. DR can help in accommodating intermittent renewable energy generation by incorporating demand-side flexibility. Hence it has drawn compelling attention from both academia and the industry in recent years which lead to the rapid growth of research on demand response including a considerable amount of scientific literature, i.e. journal articles, conference preceding, and books. This significant volume of literature is making it challenging and time-consuming for researchers and DR participators to get a synoptic and structured overview of the relevances as well as up to date information about DR. To date, several review papers have been published covering the general concept of DR and recent advances in academia as well as industrial research on DR [3]- [5].
This study aims to provide a comprehensive and structured overview of the DR domain by systematic mapping of the field. In this regard, a Bibliometric analysis has been done followed by a content analysis of the identified research themes and the broad domain. The critical contribution of the can be summarized as; The Bibliometric analysis provides an in-depth visualization and ranking of the literature under review, associated keyword trends, a conceptual map which in turn provides a swift overview of the DR research. Furthermore, a detailed content analysis of the selected scientific literature gives us the current research questions addressed by the top researchers, state of the art methods to solve those problems and also an excellent understanding to provide future research direction.
The organization of the rest of the papers is as follows; Section II provides an overview of the methodology of systematic mapping followed by Section III which includes the bibliometric analysis results. Section IV contains the analysis of the demand response research themes formulated from content analysis and Section V includes concluding remarks by outlining the future research direction of DR. Framework  Fig 1. shows the comprehensive framework of the systematic mapping study. Under the framework, an initial repository has been created containing the metadata of the relevant literature from a reliable database (SCOPUS). The initial search results have been filtered accordingly to exclude any irrelevant data or literature which does not fall under the scope of the study. Bibliometric analysis has been carried out on the refined metadata. The bibliometric analysis comprised a qualitative and quantitative analysis of bibliometric data providing insight into the research domain by highlighting the patterns of publications, associated institutions as well as journals [6].An in-depth content analysis based on the research themes identified from the bibliometric analysis provided the latest advancement, current gaps in each of the research subthemes which in turn indicate future development on the particular topic.

B. Data and Tools
The datum was retrieved from the SCOPUS database, one of the most reliable scientific citation index databases in the research community. The search terms were combined with different keywords that commonly represents the DR domain. Initial search results showed publications dated by in the 90s with the fundamental concept of DR. For the relevancy of the latest research activity, a period between "2008-2020" has been considered while searching for relevant data. Include -English Table I shows the related search term and filtration criteria on the SCOPUS database. The initial search result came with 5534 articles which were numbered down to 1377 after applying the filtration criteria. This metadata has been used to carry out the bibliometric analysis.

A. Publication Trend and Research Growth
The metadata from SCOPUS includes information such as publication year, authors, titles, abstracts, and references which can be effectively used to carry out the bibliometric analysis and information visualization. "Bibliometrix" -an R based tool [7] is used to analyze and visualize the corelationships in the metadata. One of the most important indications of the development of the research field is the number of peer-reviewed publications each year.

B. Top Source Journal
Top source journals that are publishing most frequently in the areas of Demand Response have been extracted and categorized based on the total number of publications (NP) and the total number of citations (NC). This categorization will give an impression of influential Journals to the researchers for monitoring the progress of the discipline.

C. Country Wise Analysis
In this section, the top most productive countries in terms of the number of publications are presented. The number of publications from a country reflects the attention and the overall research strengths of this country in the related field of research. Table III shows the order of the countries sorted by NP, also taking into account the NC. A total of 525 articles has been published from 1377 articles where the corresponding author is affiliated to the countries from Table  III which accounts for 38% of the total publications. China      Table V lists the highly influential papers based on the number of citations each paper got throughout the analyzed period. The main focus of the papers is on a conceptual review of DR as well as proposing optimization and control framework for cost minimization and energy saving. Keywords are generally used to emphasize the research theme as well as the content of the publications. In this study, the keywords associated with each paper have been utilized to identify a common research theme to create a conceptual map of Demand Response research. Using the bibliometrix tool, a multiple corresponding analysis (MCA) has been performed to draw the conceptual structure using a word co-occurrence network to map and cluster keyword terms that are frequently occurring in a bibliographic collection   A. Smart Homes Among the research themes, articles have been published under Smart Homes,where an emphasis has been given on the identification of suitable assets that can participate in a demand response program. Different optimization frameworks that considered the key optimization parameters such as cost, energy efficiency, user comfort have also been addressed. Pricing schemes remain a crucial challenge to identify the user's willingness to participate in DR programs. Alternatively, the estimation of uncertainties in electrical load estimation, as well as the frameworks to include the uncertainty measurement, are getting significant attention.

B. Grid Energy Management
Different DR control frameworks on grid-level energy management with distributed energy resources have been investigated to assess impact in terms of maximizing DER integration to the grid. Demand response facilitates DER integration by incorporating different resources (thermal loads, EVs, Energy Storage) and utilizing their flexible load changing capacity. However, accurate mathematical modelling of these resources is very important to fully harness their ability and integrate them into a DR framework.

C. Ancilliary Services
Several works of literature were also focused on utilizing DR for ancillary services. As the penetration of intermittent resources is increasing continuously, hence the grid stability issues such as frequency and voltage regulations are getting attention under the DR paradigm. The effect of system parameters due to external disturbances, for example, significant generation drop or large load drops are being studied while the impact of the load characteristics on system parameters has also been researched extensively in recent days.

D. Load Management
The increasing number of thermal loads, as well as EVs, brings massive challenges to the traditional grid. However, some loads such as EVs can also be utilized to inject the electrical power back to the electrical power grid. This provides an excellent opportunity to achieve active demand response and provide associated services to facilitate grid stability in the presence of intermittent resources. But how to aggregate different types of loads to participate in the DR framework, also coordinated control of these loads remains an important research question that is being addressed under the direct load management research theme.

E. Energy Market
There have been some extensive studies on the market behaviour of demand response resources. Different market structures and strategies remain the primary focus of many literature. The lack of a market mechanism in the current market structure (Day-ahead/Real-Time) remains a great barrier for demand response in most countries. Without an accurate market mechanism and demand response model, it is hardly possible to estimate the real impact of the demand response market, hence there will be a significant change in financial outcome for the consumers which has been estimated from the existing model.

F. Smart Metering
The penetration of advanced metering infrastructure enables consumers to participate more in DR programs. Since the introduction of market deregulation and market-driven pricing, utilities have been searching to equalize consumption with generation. Smart meters are providing site-specific information which helps to make decisions in response to changing prices for electricity according to the time of the day and season. Hence feature analysis of smart meter data has been getting significant research attention. Alternatively, intelligent technologies such as Non-intrusive load monitoring (NILM) are getting popular for device-level detection. As more appliances are now becoming intelligent i.e. IoT enabled, it opens up a new dimension where research has been done on identifying an individual device status to facilitate cost-effective scheduling under the DR program.

V. CONCLUSION
In this paper, a comprehensive mapping of demand-side response has been carried out under the smart grid paradigm. Demand response has shown to be efficient in mitigating many grid operational challenges. Though there have been numerous research outcomes on demand response, there are still some challenges and research questions that need further attention, • From the study, it is evident that a lot of research has been done in both residential and commercial DR context. However, there are very few cases of DR application in an industrial setting. As industries also employ flexible controllable load, evaluation of DR potential in an industrial context will prove valuable to exploit in the long run.
• Introduction of EVs, Energy storage will create new opportunities for residential and commercial level DR. Therefore more research to facilitate these assets in DR will be required.
• User comfort and pricing remain a crucial barrier for DR participation by the consumers. Introducing new strategies and control frameworks that attracts the consumers to participate in DR can be considered as a significant research challenge.
• The effect of DR schemes in the electricity market with different levels of competition is recommended for further investigation.
• Proper mathematical modeling of assets and formulating the optimization problem with more detailed parameters for DR programs are required in the future.
• As more advanced computing technologies are being involved, analysis of higher resolution smart meter data as well as interpreting the patterns will get more attention in the future.
• Responding to cyber threats remains a big question, as more and more smart meters are being deployed proper data encryption, and introducing more security features remains a significant challenge that needs to be addressed.