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Proceedings ArticleDOI

The role of thermal storage and natural gas in a smart energy system

10 May 2012-pp 1-9

Abstract: Smart grids are considered important building blocks of a future energy system that facilitates integration of massive distributed energy resources like gas-fired cogeneration (CHP). The latter produces thermal and electric power together and as such reinforces the interaction between the gas and electricity-distribution systems. Thermal storage makes up the key-source of flexibility that allows decoupling the electricity production from the heat demand. However, smart grids focus on electricity, often disregarding the role of gas and thermal storage in overall smart energy systems. We find that the technical impact of a massive introduction of CHP on the gas-distribution network is limited in most cases, even providing opportunities to free up capacity. Taking the consumer's viewpoint, we highlight the economic importance of the thermal storage tank, which requires a thermal capacity of two to three times the hourly thermal power output of the CHP to optimize electric power production and limit thermal losses. Further increasing the storage tank size can increase the gas-distribution capacity that can be marketed by the distribution system operator, but practical constraints in terms of dedicated land area have to be considered as well.
Topics: Distributed generation (60%), Thermal energy storage (59%), Energy storage (59%), Smart grid (59%), Electricity generation (58%)

Summary (4 min read)

Introduction

  • ROBERT SCHUMAN CENTRE FOR ADVANCED STUDIES Jeroen Vandewalle, Nico Keyaerts and William D'haeseleer THE ROLE OF THERMAL STORAGE AND NATURAL GAS IN A SMART ENERGY SYSTEM EUI Working Papers RSCAS 2012/48 ROBERT SCHUMAN CENTRE FOR ADVANCED STUDIES Loyola de Palacio Programme on Energy Policy.

The Role of Thermal Storage and Natural Gas in a Smart Energy System

  • JEROEN VANDEWALLE, NICO KEYAERTS AND WILLIAM D'HAESELEER EUI Working Paper RSCAS 2012/48.
  • This text may be downloaded only for personal research purposes.
  • Additional reproduction for other purposes, whether in hard copies or electronically, requires the consent of the author(s), editor(s).
  • If cited or quoted, reference should be made to the full name of the author(s), editor(s), the title, the working paper, or other series, the year and the publisher.
  • ISSN 1028-3625 © 2012 Jeroen Vandewalle, Nico Keyaerts and William D'haeseleer Printed in Italy, September 2012 European University Institute Badia Fiesolana I – 50014 San Domenico di Fiesole (FI) Italy www.eui.eu/RSCAS/Publications/ www.eui.eu cadmus.eui.eu.

Robert Schuman Centre for Advanced Studies

  • The Robert Schuman Centre for Advanced Studies , created in 1992 and directed by Stefano Bartolini since September 2006, aims to develop inter-disciplinary and comparative research and to promote work on the major issues facing the process of integration and European society.
  • The Centre is home to a large post-doctoral programme and hosts major research programmes and projects, and a range of working groups and ad hoc initiatives.
  • Details of the research of the Centre can be found on: http://www.eui.eu/RSCAS/Research/.
  • Research publications take the form of Working Papers, Policy Papers, Distinguished Lectures and books.
  • The EUI and the RSCAS are not responsible for the opinion expressed by the author(s).

Loyola de Palacio Energy Policy Chair

  • The Loyola de Palacio Energy Policy Chair was created in October 2008 at the RSCAS in honour of Loyola de Palacio, former Vice President of the European Commission and Commissioner for Energy and Transportation in the Prodi Commission.
  • Professor Jean-Michel Glachant is the holder of the Chair.
  • The Chair focuses on the fields of energy economics, law, regulation, as well as geo-politics.
  • It addresses topics such as the achievement of the EU internal energy market; sustainable energy systems and the environment; energy security of supply; the EU model of energy regulation; the EU energy competition policy; the EU policy towards carbon free energy systems in 2050.
  • The series of working papers aims at disseminating the work of academics on the above-mentioned energy policy issues.

For further information

  • Loyola de Palacio Energy Policy Chair Nicole Ahner (scientific coordinator) Email contact: Nicole.Ahner@eui.eu Robert Schuman Centre for Advanced Studies European University Institute Via delle Fontanelle, 19 I-50016 San Domenico di Fiesole (FI) Fax: +39055 4685755 http://www.loyola-de-palacio-chair.eu.

Keywords

  • Smart Grids, Cogeneration, Natural Gas, Energy Storage.

Nomenclature

  • C thermal capacity storage tank kWh cp thermal capacity of water J/kg.
  • K electric power CHP unit kW gas demand kWh/h maximum in reference gas demand kWh/h heat demand kWh/h L loss factor kWh/h m hourly average boiler modulation - pe electricity price €/kWh pg gas price €/kWh thermal (dis)charging power kWh/h thermal power condensing boiler kW thermal power CHP kW R ratio thermal to electric power CHP - s CHP on/off variable - t time h T temperature K V volume of the storage tank m³ x storage tank energy contents kWh xMin minimum energy level storage tank kWh xMax maximum energy level storage tank kWh.

Greek symbols

  • Electric efficiency CHP unit thermal efficiency CHP unit thermal efficiency condensing boiler thermal efficiency storage tank peak increase gas demand gas demand peak increase.

Subscripts

  • Smart grids are considered as an important next step towards a reliable and sustainable energy provision [1, 2].
  • CHP is a very interesting technology because of its efficient fuel utilization and the possibility to interact with the electricity grid.
  • With thermal storage, the heat production can be decoupled from the heat demand, giving flexibility to produce electricity based on incentives from the electricity system.
  • The impact depends on the exact gas demand of the CHPs, and these depend on the use of thermal storage and the interaction between the gas and electricity distribution systems.
  • The aim of this paper is to focus on the gas distribution system and investigate how the smart grid with massive CHP penetration and thermal storage affects it, or better, how these elements of a smart energy system interact.

II. Models and Equations

  • This part describes the models and equations used in this work.

A. Assumptions

  • The heating systems of a number of households will be simulated to see what their resulting gas demand is.
  • To find the gas demand of a household, the heating system, including the CHP unit, is simulated, such that if fulfills an imposed heat demand.
  • 1. Fig. 1. Schematic representation of the work flow.
  • The heat to electric output ratio of the CHP is assumed to be 4:1 and the fuel utilization ratio amounts to 95%.
  • The authors suppose a perfectly stratified thermal storage tank.

B. The Heating System Simulation Model

  • The heating system that will be modelled consists of a CHP unit with a separate auxiliary boiler and a thermal storage tank, see Fig.
  • The term adapted annual gas cost is used here because the revenues from the produced electricity are subtracted from the annual gas bill.
  • Equation (2) describes the heat balance: for every hour t, the heat demand (kWh/h) must be met either by the boiler, the CHP or the storage tank.
  • The (dis)charging power of the storage tank during hour t is the variable (kWh/h).
  • The losses due to the temperature difference between the low temperature tank and the surrounding air temperature are denoted by (kWh/h). (4) The constraints are: the storage tank starting energy equals the minimal operating energy (Eq. 5), the (dis)charging power is limited to 50 kW (Eq. 6), the hourly average modulation of the boiler mt must be in the interval [0,1] (Eq. 7) and the stored energy inside the storage tank must always be more than the minimal and less than the maximal operational capacity (Eq. 8). (5) (6) (7) (8).

C. Sizing of the CHP and the storage tank

  • The CHP cannot be designed to meet the maximum heat demand because it would be switched on and off very frequently, leading to transient behavior that may shorten the lifetime and the possible energy savings [5].
  • 3. Next, the rectangle with the largest area that can be subscribed by the load-duration diagram is determined.
  • The thermal capacity C (kWh) and the volume V (m³) of the tank have the following relation: (9) where is the density of water, is the thermal capacity of water and is the temperature difference between the high and the low temperature part of the storage tank.
  • The Relative Storage Capacity can be calculated as: (10) According to this method, the CHP in this example should have a thermal output of 4.15 kW and will be on for 2260 hours per year.
  • During spring, autumn and especially the summer, the CHP is much more responsive to the electricity price levels because it will not be on all day.

III. Technical Impact on the Gas Grid

  • This section examines the technical impact of cogeneration on the gas distribution network.
  • The most important parameter to check this is the total gas demand of all households connected to the grid, which should not be higher than the capacity of the gas network in order to be able to supply the households.
  • First, a theoretical maximum impact is derived, followed be a more practical maximum peak demand.
  • The scenarios in this part assume a massive introduction of CHP.
  • Hence, all users are equipped with CHP and thermal storage.

A. Theoretical maximum peak demand

  • The theoretical ‘worst case’ scenario is when all customers act exactly the same; there is no averaging effect and all gas demand peaks will therefore occur at the same time.
  • Next, the authors derive what the maximum increase in peak demand would be in the absence of storage.
  • The maximum peak demand will occur on the coldest day of the year.
  • 3. So, a part of the heat demand will be covered by the CHP and the remaining part by the auxiliary boiler.
  • The ‘theoretical limit’ for the peak gas demand increase = 14% can be regarded as being independent of the buffer size and the electricity price.

B. Practical maximum peak demand

  • In Fig. 6, the authors show how the peak increase changes with the RSC.
  • The latter observation is in contrast with the findings from part A of this section, where increasing the storage tank size beyond the reference value did not have much influence.
  • This outcome occurs because the actual profiles can differ very much from the average profile, such as the one depicted in Fig.
  • The main observation here is that a massive introduction of CHP does not lead to a peak increase for RSC values of 2.3 and higher, but to a peak demand decrease.
  • The grey line represents the average reference gas demand.

C. Conclusions on the technical impact on the gas network

  • It can be concluded that, for their cases and assumptions considered, a massive introduction of CHP would not lead to general technical problems, as long as the thermal storage tanks have a capacity of two or more times the hourly thermal output of the CHP.
  • Local problems in congested pipelines could occur, especially in neighborhoods with similar users.
  • The authors consider a peak demand increase of 14% as a limit, i.e. when all users act exactly the same, which is not likely to occur.
  • Increasing the storage size beyond an RSC of 2.3 further decreases the gas demand peak, creating the opportunity to free up capacity in the gas distribution network.
  • 9 impact on the peak demand is negligible.

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ROBERT SCHUMAN CENTRE FOR ADVANCED STUDIES
Jeroen Vandewalle, Nico Keyaerts and William D'haeseleer
THE ROLE OF THERMAL STORAGE AND NATURAL GAS IN
A SMART ENERGY SYSTEM
EUI Working Papers
RSCAS 2012/48
ROBERT SCHUMAN CENTRE FOR ADVANCED STUDIES
Loyola de Palacio Programme on Energy Policy


EUROPEAN UNIVERSITY INSTITUTE, FLORENCE
ROBERT SCHUMAN CENTRE FOR ADVANCED STUDIES
LOYOLA DE PALACIO PROGRAMME ON ENERGY POLICY
The Role of Thermal Storage and Natural Gas in a Smart Energy System
JEROEN VANDEWALLE, NICO KEYAERTS AND WILLIAM D'HAESELEER
EUI Working Paper RSCAS 2012/48

This text may be downloaded only for personal research purposes. Additional reproduction for other
purposes, whether in hard copies or electronically, requires the consent of the author(s), editor(s).
If cited or quoted, reference should be made to the full name of the author(s), editor(s), the title, the
working paper, or other series, the year and the publisher.
ISSN 1028-3625
© 2012 Jeroen Vandewalle, Nico Keyaerts and William D'haeseleer
Printed in Italy, September 2012
European University Institute
Badia Fiesolana
I 50014 San Domenico di Fiesole (FI)
Italy
www.eui.eu/RSCAS/Publications/
www.eui.eu
cadmus.eui.eu

Robert Schuman Centre for Advanced Studies
The Robert Schuman Centre for Advanced Studies (RSCAS), created in 1992 and directed by Stefano
Bartolini since September 2006, aims to develop inter-disciplinary and comparative research and to
promote work on the major issues facing the process of integration and European society.
The Centre is home to a large post-doctoral programme and hosts major research programmes and
projects, and a range of working groups and ad hoc initiatives. The research agenda is organised
around a set of core themes and is continuously evolving, reflecting the changing agenda of European
integration and the expanding membership of the European Union.
Details of the research of the Centre can be found on:
http://www.eui.eu/RSCAS/Research/
Research publications take the form of Working Papers, Policy Papers, Distinguished Lectures and
books. Most of these are also available on the RSCAS website:
http://www.eui.eu/RSCAS/Publications/
The EUI and the RSCAS are not responsible for the opinion expressed by the author(s).
Loyola de Palacio Energy Policy Chair
The Loyola de Palacio Energy Policy Chair was created in October 2008 at the RSCAS in honour of
Loyola de Palacio, former Vice President of the European Commission and Commissioner for Energy
and Transportation in the Prodi Commission. It promotes research in the area of energy policy. It is
funded by contributions from donors. Professor Jean-Michel Glachant is the holder of the Chair.
The Chair focuses on the fields of energy economics, law, regulation, as well as geo-politics. It
addresses topics such as the achievement of the EU internal energy market; sustainable energy
systems and the environment; energy security of supply; the EU model of energy regulation; the EU
energy competition policy; the EU policy towards carbon free energy systems in 2050.
The series of working papers aims at disseminating the work of academics on the above-mentioned
energy policy issues.
For further information
Loyola de Palacio Energy Policy Chair
Nicole Ahner (scientific coordinator)
Email contact: Nicole.Ahner@eui.eu
Robert Schuman Centre for Advanced Studies
European University Institute
Via delle Fontanelle, 19
I-50016 San Domenico di Fiesole (FI)
Fax: +39055 4685755
http://www.loyola-de-palacio-chair.eu

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Abstract: When evaluating the environmental impact of small-scale cogeneration facilities, two important boundary conditions are often overlooked. Firstly, cogeneration units are mostly considered as stand-alone facilities, although, in reality, they will be part of a system that may also contain a thermal-storage tank and back-up boiler. Secondly, usually mainly static and simplified methods are used to calculate the possible reduction of CO2 emissions. In this paper, these issues are discussed in two parts. The dimensioning of cogeneration facilities to fulfil a certain heat demand and the impact of thermal-storage tanks on the operational behaviour of these units are dealt with. It is shown that the use of thermal-storage tanks prolongs the yearly operation time of a CHP facility and allows the cogeneration unit to operate more continuously. Also, it is clarified how to interpret thermal load-duration diagrams in a correct way. Furthermore, the impact of thermal storage on the overall CO2 emissions is investigated. Hereby, the interaction with the expansion of the central power system and the annual use of the cogeneration units are two important parameters. Using a small thermal-storage device causes the net reduction of CO2 emissions, in comparison with a reference scenario without additionally installed cogeneration, to be almost three times higher compared to the case without heat buffer. Finally, it is shown that the operational behaviour of multiple small-scale cogeneration units can be approximated by the behaviour of one large fictitious unit for the determination of the net reduction of CO2 emissions.

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TL;DR: An optimisation method based on mixed integer linear programming (MILP) for the management of local heat supply systems with CHPs, heating boilers and thermal storages is developed and allows the production of thermal and electric energy with a maximal benefit.
Abstract: Within several projects we investigated grid structures and management strategies for active grids with high penetration of renewable energy resources and distributed generation (RES & DG). Those ”smart grids” should be designed and managed by model based methods, which are elaborated within these projects. Cogeneration plants (CHP) can reduce the greenhouse gas emissions by locally producing heat and electricity. The integration of thermal storage devices is suitable to get more flexibility for the cogeneration operation. If several power plants are bound to centrally managed clusters, it is called “virtual power plant”. To operate smart grids optimally, new optimisation and model reduction techniques are necessary to get rid with the complexity. There is a great potential for the optimised management of CHPs, which is not yet used. Due to the fact that electrical and thermal demands do not occur simultaneously, a thermally driven CHP cannot supply electrical peak loads when needed. With the usage of thermal storage systems it is possible to decouple electric and thermal production. We developed an optimisation method based on mixed integer linear programming (MILP) for the management of local heat supply systems with CHPs, heating boilers and thermal storages. The algorithm allows the production of thermal and electric energy with a maximal benefit. In addition to fuel and maintenance costs it is assumed that the produced electricity of the CHP is sold at dynamic prices. This developed optimisation algorithm was used for an existing local heat system with 5 CHP units of the same type. An analysis of the potential showed that about 10% increase in benefit is possible compared to a typical thermally driven CHP system under current German boundary conditions. The quality of the optimisation result depends on an accurate prognosis of the thermal load which is realised with an empiric formula fitted with measured data by a multiple regression method. The key functionality of a virtual power plant is to increase the value of the produced power by clustering different plants. The first step of the optimisation concerns the local operation of the individual power generator, the second step is to calculate the contribution to the virtual power plant. With small extensions the suggested MILP algorithm can be used for an overall EEX (European Energy Exchange) optimised management of clustered CHP systems in form of the virtual power plant. This algorithm has been used to control cogeneration plants within a distribution grid.

118 citations


Journal ArticleDOI
Abstract: This paper considers the effect that different hot water storage tank modelling approaches have on the global simulation of residential CHP plants as well as their impact on their economic feasibility. While a simplified assessment of the heat storage is usually considered in the feasibility studies of CHP plants in buildings, this paper deals with three different levels of modelling of the hot water tank: actual stratified model, ideal stratified model and fully mixed model. These three approaches are presented and comparatively evaluated under the same case of study, a cogeneration plant with thermal storage meeting the loads of an urbanisation located in the Bilbao metropolitan area (Spain). The case of study is simulated by TRNSYS for each one of the three modelling cases and the so obtained annual results are analysed from both a First and Second-Law-based viewpoint. While the global energy and exergy efficiencies of the plant for the three modelling cases agree quite well, important differences are found between the economic results of the feasibility study. These results can be predicted by means of an advanced exergy analysis of the storage tank considering the endogenous and exogenous exergy destruction terms caused by the hot water storage tank.

115 citations


"The role of thermal storage and nat..." refers background in this paper

  • ...06 €/kWh unless ment We suppose a perfectly stratified thermal s means that the hot water does not mix with the tank, and that the thermal conductance of From an energy point of view, the perfectly gives good results compared to the actual which is more complex but describes the storage more accurately [9]....

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  • ...[9] A....

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