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

TL;DR: In this paper, the authors 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.

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.

Summary

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.

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.

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.
• 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).

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].
• 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.
• 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.

Figures

Fig. 3. An example of CHP sizing with the largest rectangle under the load-duration curve.

Fig. 6. The peak gas demand increase caused by CHP relative to the original peak demand as a function of the relative storage capacity, averaged per user. The actual peak increase depends on the case, but generally decreases with the relative storage capacity. From an RSC of 7 on, the

Fig. 4. The gas demand for a household with an average heat demand. The grey line is the original, reference gas demand without CHP on a typical cold day. The thick black line is the gas demand for this household with CHP without storage. The gas demand when a 100 L of thermal storage is

Full text

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

Frequently asked questions

Q1. What are the contributions mentioned in the paper "The role of thermal storage and natural gas in a smart energy system" ?

Vandewalle et al. this paper showed that the technical impact of a massive introduction of cogeneration on the gas-distribution network is limited in most cases, even providing opportunities to free up capacity. 

Q2. What have the authors stated for future works in "The role of thermal storage and natural gas in a smart energy system" ?

Future work will include the analysis of how to determine the local effects in the gas distribution network. 

Q3. how does a peak demand increase increase gas storage?

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. 

Q4. What is the impact of CHP on the gas distribution system?

In general, the average gas demand is expected to increase with a rising penetration level of CHP, potentially leading to physically congested pipelines. 

Q5. What is the impact of the CHP on the gas distribution 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. 

Q6. What is the effect of a massive introduction of CHP?

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. 

Q7. Why is the term adapted annual gas cost used?

The term adapted annual gas cost is used here because the revenues from the produced electricity are subtracted from theannual gas bill. 

Q8. What is the role of gas and thermal storage in smart energy systems?

smart grids focus on electricity, often disregarding the role of gas and thermal storage in overall smart energy systems. 

Q9. What was the funding for this work?

This work was partly funded by the research project on ‘local intelligent networks and energy active regions’ (LINEAR)supported by the Flemish agency for innovation through science and technology (IWT). 

Q10. Why is CHP a dispatchable source of electric power?

contrary to most renewable DER, CHP is a dispatchable source of electric power because of the continuous availability of gas as its fuel. 

Q11. What is the energy balance for the CHP?

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. 

Q12. What is the economic rationale of the customer to use CHP?

Besides studying the technical impact on the gas distribution network, the economic rationale of the customer to use CHP should be investigated as that analysis sheds light on how the gas demand can look like if the role of thermal storage is taken into account. 

Q13. What is the way to keep the CHP running all night?

4. For some heat demand profiles, a RSC of 2.3 is not sufficient to keep the CHP running all night; a higher RSC value would therefore be needed. 

Q14. What is the effect of the buffer size on the peak gas demand?

It can also be noted in Fig. 4. that the line for 100 L shows a fallback of gas demand by night because the buffer is not large enough to store enough heat from the CHP to keep it on all night. 

Q15. How does the peak increase change with the RSC?

The actual peak increase is case dependent— that is why Fig. 6 is not a smooth curve—but in general, it decreases with increasing relative storageJeroen Vandewalle, Nico Keyaerts and William D'haeseleer8capacity, even when an RSC of 2.3 is exceeded, up to an RSC of 7. 

Q16. What is the role of the gas distribution network in a smart energy system?

The natural gas distribution network is represented by a hypothetical model, disregarding the pressure losses and the location of the consumers in the network. 

Q17. What is the effect of increasing the buffer size beyond the optimal value?

Increasing the buffer size beyond the optimal value is also be sub-optimal for the customer due to the extra thermal losses of the tank.