XIII-04: Cost Benefit study on the building secondary network for improving DH performance

Hello, I'm Pascal stavart from the EcoLeademy in Dubai. Antoine Fabre and I are going to present you the Biyung substation project funded by the International Energy Agency as part of the district heating and cooling program. Our project concerns the cost benefit study on the building secondary network systems for for improving district heating performance. This project was carried out with engie, a major operator of heating and cooling networks in Europe, an ivl, a Swedish environmental research institute.

To begin, let's introduce the context of this study. To improve the performance of distillcating, it's necessary to reduce the water return temperature. Indeed, it allows to reduce heat losses, pump consumption and to increase the use of low temperature renewable energy sources. Some references from the literature are given on this slide. On the left side, Gad and Werner found that in existing networks only 26% of substations and were operating correctly. A malfunctioning substation is responsible for the high return temperature on primary side. On the right side of this slide, another study on existing networks identify the origin of malfunctions. The secondary network systems represent 33% of malfunctions. The secondary systems are a key point for improving the performance of district kitting. But it's a difficult problem to tackle since the district operators are often different from building operators.

The objectives of this project are first to bring out some recommendations to improve the design and operation of the secondary heating systems for reducing return temperature and secondly to identify heat tariff models to foster building owners to improve the building side hydronic systems. The project was divided into several tasks. Firstly, a state of the art was undertaken to define the most common secondary systems in France, which offer great potential for improvement. We selected also rate of detections which can be carried out on the common low efficient secondary systems. Secondly, we modeled generic heating networks including the the secondary networks in the connected buildings. The developed modeling platform allows us first to create four diagnosis indicators on secondary side and secondly to assess the energy impact on the networks of rate of detection on the secondary side. Finally, we studied our motivational tariffs can encourage building owners to undertake renovation works on their hydronic systems.

Based on the literature review, we decided to investigate two different scenarios. The first one is about a third generation distributed network supplying by geothermal energy and gas. The supply Temperature is around 90 degrees. The second scenario is about a new generation distributed network supplying by shallow geothermal energy, heat pump and gas. Here, the supply temperature is around 50 degrees.

Indeed, according to the scenarios, the rate of heat action on the building side may not have the same impact. Then, several Retrofit actions are studied two substation architecture, one with two heat exchangers in a parallel configuration and another one with only one global heat exchanger. We also studied two types of radiator, a high temperature one and a low temperature one. We also include two types of radiator controls with two or three way valves coupled with a constant or variable flow rate pumps. To finish, we take into account also three way to supply domestic hot water in particular with or without a storage tank.

In order to study all these retrofit actions, a generic district heating network model has been developed. The dynamic time wide rake model created with the software Demola Modelica encompasses all the part of distributing network from the E generation plants to the building, including the control system. Nevertheless, before considering to retrofit the systems, it is necessary to identify which are the less efficient substation and then to check if we can improve in them by correcting malfunction. In the literature, several indicators are available, but they are not enough specific to detect the origin of the malfunction. So we decided to study four.

A primary side valve blocked, the fouling of heat exchanger, a secondary side valve blocked and also the detection of an height domestic artwater recirculation return temperature.

The first malfunction deals with a primary side valve blocked on an opening position. The specific signature of this fault is an instability of the flow rate passing through the valve. So we propose to follow the relative variation of the mass flow rate through the time. If the relative variation is up to 15% more than four times a day, there is a faulty valve. The function is about the heat exchanger fouling. As you know, the fouling increases the thermal resistance of the heat exchanger. The indicator is based on the evolution of the thermal resistance versus a mass flow rate ratio. This indicator only works for plate heat exchanger.

The idea of this indicator is to compare the evolution of thermal resistance during operation with with a reference case. The reference case is the evolution of thermal resistance versus the mass freight ratio after a cleaning of the heat exchanger. Because this evolution is linear and we consider that the performance are significantly degraded when the thermal resistance during operation is superior than 1.4 times the thermal resistance without fooling for a given mass flow rate ratio.

The next malfunction deals with a secondary side valve blocked, the valve controlling the flow passing through the radiator. When this valve is blocked on an opening position, the variations of the radiator's DTLM are smoothed. So we propose an indicator based on this specific behavior. When the relative variation of the radiator DTLM is lower than 10% of the malfunction is detected. The last malfunction is about a too high domestic hot water recirculating temperature caused by an unsuitable recirculation flow rate. On the figure we plot the primary return temperature versus the primary supply temperature. The blue dots stand for a well designed recirculation line, whereas the orange one represents a recirculation line with a bad design.

Next, by assuming a recirculation line return temperature and a fixed ratio between the recirculant flow rate and the daily average demand flow rate, it is possible to plot the ISO effectiveness line. If the dots are upper than the gray line of an effectiveness of 0.8, a malfunction is detected. Indeed, an effectiveness of 0.8 for heat exchanger supplying the miscalculated water is is a reasonable effectiveness.

Once the malfunctions are detected and corrected, retrofit actions on the secondary side can be considered.

Based on a large set of simulations. The performance gains of all the retrofit action combinations are summarized in a decision tree. The decision tree is split into four layers. Each layer corresponds to a choice between several options in terms of performance. The best option is represented by a green line, whereas the red line stands for the worst options.

The return temperature gained from a retrofit action to a better one is represented by a black dot line.

Thus, the first layer is the choice between an high temperature radiator or a low temperature one. The replacement of a high temperature radiator by a low temperature radiator which is the best solution Allow to gain up around 15 degrees Celsius.

Next, if we consider that high temperature radiators are installed, the next choice is about the substation architecture. Here the pile architecture is the best solution. The third layer is about the choice of the secondary flow control. The worst solution is a radiator's flow supplied by a constant flow rate pump controlled through a three way valve.

A solution where the constant flow rate pump is replaced by a variable flow rate pump is a better solution, but the best solution is to set up a variable flow rate pump and a two way valve.

To finish. The last layer is the choice of the domestic hot water supply architecture. The instantaneous solution without storage tanks appears to be the best solution. Here.

The same decision tree has been made. If the first choice is to use low temperature radiators, the best solutions are similar to the previous case. So we need to use two way valves with a viable macerate pump, a substation in a parallel configuration.

And to supply domestic hot water without storage tank.

The last task of this project deals with tariff models. First the cost of retrofit actions and secondary side are studied. Then, based on a literature review, motivational tariff are studied. There is a trend in Europe to upgrade price models by inclusion of different motivations for lowering the return temperature.

In the figure presented here, 36 retrofit actions are plotted even the ones with a negative effect with an increase of return temperature. On the x axis you can find the decrease of return temperature on primary side. On the y axis you can find the cost of undertaking the retrofit actions. The cases with one action are marked in red, two actions are marked in yellow, three actions are marked in blue and floor action are marked in dark purple.

The reference case is a unique heat exchanger in the substation for space heating and domestic hot water and in the secondary side high temperature radiators with a constant flow rate pump and three way valves and a tank for domestic water. If only one action is undertaken, the best effect is to use two way valves and variable flow rate that we call in the following Efficient flow if only two actions are undertaken, the best effect is to combine efficient flow with low temperature radiators. If three actions are undertaken, the best effect is to combine efficient flow with low temperature radiators and parallel heat exchangers. If four actions are undertaken, the best effect is to add to the previous case an instantaneous domestic hot water preparation. Payback times for the best combination of actions are presented on this slide. For a low gas price and a high gas price, the best actions presented in the previous slide are shown here. In case of one to four retrofit actions as shown in this figure, a situation with a high level of natural gas prices leads to significantly lower payback time for the actions a about one year, whereas gas prices at lower levels yields payback times of around six to nine years.

To conclude on this part, the best option to install are first to have an efficient flow by installing variable flow rate pump combined with two way valves. Then introduce low temperature radiators is very useful for increasing distributing performance provided that the building envelope has been retrofitted or else by increasing the heat exchanger surface of new radiators. Then parallel heat exchanger in the substation for space heating and domestic hot water should be implemented. Finally, it's advisable to remove water tank and replace them by instantaneous domestic hot water preparation to foster the improvement of the sunglary side of substations. There are several ways to incorporate a motivational component in the district heating price models. First A flow component is an indirect way to target low return temperature as an increased flow low corresponds to high return temperature. It can also be useful to add an incentive tariff for reducing the return temperature on the primary side to below the design value. The incentive tariff can be based on an average temperature reduction in the substation using monthly values with a certain bonus in euro per degrees. A second incentive tariff could be applied for using return line heats. The aim is to encourage customers to install low temperature system such as flow heating and instantaneous domestic hot water preparation. A price model encouraging substation efficiency can be suggested as shown here with first a fixt tariff as a subscription for a certain capacity and connection distance as well, then a variable tariff linked to energy consumed and the water volume consumed as well, and finally an incentive tariff. In other words, the monetary gain that the consumer gets from improving the performance of the secondary unit. As a conclusion, the performance and cost of 36 retrofit action combinations on secondary side have been assessed on a numerical bench test. The most interesting retrofit actions are efficient secondary flow rate control and replacing high temperature radiators by low temperature radiators. The payback time of the most interesting retrofit actions with high gas prices is around one year. However, their simulation results must be consolidated by by institut measurement. From this study some recommendations can be formulated. It seems important to engage a dialogue with customers about the interests of motivational tariffs. The client as provider of wastage and the client flexibility will help to develop these motivational tariffs.

In order to continuously improving the performance of existing networks, it's essential to first set up a performance monitoring on primary and secondary sites to suppress faults on the system. Finally, retrofit actions should be undertaken to improve the design performance.