Elsevier

Journal of Energy Storage

Volume 15, February 2018, Pages 350-358
Journal of Energy Storage

Integration of heat storage system into district heating networks fed by a biomass CHP plant

https://doi.org/10.1016/j.est.2017.12.010Get rights and content

Abstract

Biomass Combined Heat and Power (CHP) plants connected to district heating networks (DHN) are recognized as a very good opportunity to increase the share of renewable sources into energy systems. However, as CHP plants are not optimized for electricity production, their operation is profitable only if a sufficient heat demand is available throughout the year. On the other hand, these plants often work for baseline operations and back-up boilers are used to supply the peak demand. To extend the use of the CHP plants and reduce costs, conventional fuel use and emissions, it is proposed to study the feasibility of using the DHN itself or additional high temperature heat storage as retrofit of an existing CHP plant.

This work is based on a simple and effective methodology that provides accurate estimations of economic, environmental and energetic performances of CHP plants connected to district heating networks. The focus is performed on the integration of the heat storage as retrofit of existing DHN considering the local policies.

The DHN of the University in Liège (Belgium) is used as an application framework to demonstrate the effectiveness of the selected approach. The potential energy, pollutant emissions savings and resulting energy costs are estimated and the current policy limitations will be discussed.

Introduction

Biomass Combined Heat and Power (CHP) plants connected to district heating networks (DHN) are recognized nowadays as a very good opportunity to increase the share of renewable sources into energy systems and a convenient way to supply heat to a large number of individual using a unique central heating plant [1,2]. Indeed, large CHP plants combine high conversion efficiency, high availability and low operation costs. From an environmental point of view, the use of biomass allows a significant reduction of CO2 emissions compared to the use of natural gas due to their low emission levels [3].

The economical optimum of (biomass) CHP plant coupled to a DHN consists in using the CHP plant for the base load as often as possible while keeping the back-up boilers for the peak load. Yet, in practice, the heat demand can widely vary during the day due to a lot of non-controlled variables such as weather conditions, users’ habits…. Therefore to extend the use of the biomass CHP and maximize its average efficiency, one solution is to consider a heat storage system to supply heat during the peak load by storing the unused energy produced by the plant from the low heat demand periods [4]. Indeed this solution is generally used to make CHP or biomass boilers more flexible leading to a better environmental and economic efficiencies [[5], [6], [7], [8]]. Several technologies of heat storage are commercially available but this contribution is not intended to be a review of these technologies and the reader interested can refer to [4,9,10]. The focus is to assess the impact of the integration of a heat storage system on an existing DHN. Here the thermal energy storage considered uses water as medium to store the thermal energy inside an insulated tank or inside the DHN itself by a dedicated control of the energy supplied to the DHN. This last solution has the advantage to limit drastically the investment costs of the heat storage system but its capacity is limited by the DHN size and the control strategy.

The purpose of the present study is to determine the best integration of a heat storage system which minimizes the cost of heat of a heating plant feeding a DHN while minimizing its environmental impacts and considering or not the current Belgian policies (green certificate and CO2 carbon price). To achieve this aim, a previous developed and validated model [11] of a biomass CHP plant supplying a district heating network taking into account the heat losses and a hot water heat storage system is used. On the other hand, the DHN itself is considered to store energy by using higher water temperatures while considering the related higher heat losses. Moreover, the dynamic model of the DHN was developed to assess the influence of the variable heat demand and the temperature level on the CHP plant efficiency and on the biomass consumption. In order to improve the profitability of the CHP plant, several scenarii of hot water heat storage (short and long term) and heat demand profile are investigated while the integration of the heat storage coupled to a CHP plant has to be carefully studied [12,13]. Indeed this contribution points out heat storage systems could be not economically profitable depending on the subsiding policies allocated to the heating plant. Finally, the Belgian subsiding policy is studied in that context.

The considered approach is performed using the measurements from an existing biomass CHP plant connected to a district heating network installed on the Campus of the University of Liège (Belgium). Due to current electricity regulation and the studied CHP plant constraints (stop to full load cycles take more than 24 h), the SPOT market and the variable electricity selling price won’t be analyzed in this study.

Section snippets

Problem statement

Thermal energy storage systems will play an important part of the energetic transition. Indeed, they can be integrated into renewable systems which are, by nature, more fluctuating than conventional power and heating plants. On another hand, the integration of heat storage system into a DHN allows the operator to optimize the heating plant. For example, the use of the cheapest production unit can be used continuously when heat storage system is available and well designed. However, the heat

Simulation model

For reasons of simplicity, a brief description of the model is provided in this section but more detailed information can be found in previous works [11,19]. The complete simulation model of the plant is an aggregation of basic components modeled by a zero-dimensional (i.e., input-output) approach verifying the conservation of mass, energy and momentum. The biomass combustion model is handled through a general biomass composition CmHnOxNySz (where the subscripts are the ratio between wet basis

Application test case

The aforementioned simulation model is applied to a typical district heating application available on the University campus in Liège (Belgium). The installed network has a total length of 10 km and distributes pressurized hot water at about 125 °C, on average, to approximately 70 buildings located in the University campus representing a total heat area of about 470 000 m2. Buildings are very different in nature namely, classrooms, administrative offices, research centers, laboratories and a

Neglecting the belgian subsidies

Due to the wide variety of CHP regulation [28], it is proposed to not consider the specific Belgian subsidies in this subsection. Only the selling of CO2 can be considered at a mean price of 5€ per ton [29]. In the nominal case of the global system (without heat storage), the cost of heat is about 77 €/MWh (Fig. 4–black circles). An optimized heat storage volume of 400 m3 leads to a COH reduction of 0.25 €/MWh. However due to heat storage investments costs, the payback time is 13 years which is

Conclusions and perspectives

Thermal energy systems are generally used in combination with CHP plant or boilers to use them at their nominal efficiencies as long as possible and reduce costs and energy consumption. In this contribution, a retrofit of an existing system composed of a biomass CHP plant connected to a DHN is investigated. The study aims to optimize the heat storage volume which can be connected to the plant to maximize the energetic, environmental and economic benefits. This retrofit analysis is based on a

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