The role of thermal storage and natural gas in a smart energy system
Summary (4 min read)
- 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.
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- 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.
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- 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.
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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.
- Smart Grids, Cogeneration, Natural Gas, Energy Storage.
- 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.
- Electric efficiency CHP unit thermal efficiency CHP unit thermal efficiency condensing boiler thermal efficiency storage tank peak increase gas demand gas demand peak increase.
- 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.
- 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 .
- 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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Cites background from "The role of thermal storage and nat..."
...The impact of a large-scale introduction of CHP on the gas network was investigated in , where the importance of a thermal storage tank was stressed to limit the impact on the gas demand....
Cites background from "The role of thermal storage and nat..."
...Nevertheless, as well known in literature , the energy management cannot be based only on the management of the electrical energy but has to include others energy sources, like gas and district heating....
"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 ....
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