Consumer Unit for Low Energy District Heating Net

Abstract

A low energy/ low temperature consumer installation is designed and analyzed. The consumer type is a low energy single family house 145 m with annual energy consumption in the range of 7000 kWh, incl. domestic hot water in a 2800 degree day climate. The network is an extreme low temperature system to reduce heat loss in the network. The consumer’s installation is a unit type with an accumulation tank for smoothing the heat load related to the domestic hot water. The building heat load is delivered by an under-floor heating system. The heavy under-floor heating system is assumed to smooth the room heat load on a daily basis, having a flow temperature control based on outdoor climate. The unit is designed for a near constant district heating water flow. The paper describes two concepts. The analyses are based on TRNSYS (Klein et al., 2006) simulation, supplied with laboratory verification of the critical accumulator. INTRODUCTION As one part of the Danish governmentally founded project “Development and Demonstration of Low Energy District Heating for low energy Buildings”, EFP2007, a new unit concept for a low energy district heating systems has been investigated and designed. In Denmark new building regulations has been introduced which largely reduce the energy consumption. For single family houses this means, that the network heat loss may be a very significant part of the total energy consumptions. Nevertheless district heating has advantages compared to other sources, especially the possibility to use waste heat, combined heat and power, difficult fuels including flue gas cleaning etc. Therefore it is interesting to investigate if a system minimizing the network heat loss and minimizing the water flows in the pipes is feasible. Such a low temperature system will also increase the potential for use of waste heat and improve electricity efficiency in the case of combined heat and power. The project deals with the a system including network, building, consumer installation and heating system in the house for room heating and heating of domestic hot water. This paper deals with the consumer part of the system. The design criteria allow for small network dimensions and a very low heat loss and are as follows: • Flow temperature from net: 50 °C • Return to net approx. 20 °C. • Differential pressure 0.2 to 1 bar, in some cases higher. • Design heat demand at -12 °C outdoor temperature approximately 2000 W • Design flow temperature to the floor heating system : 28 °C, at – 12 °C outdoor temperature • Design domestic hot water temperature: 4045 °C • Design cold water temperature 10 °C Especially the pipes from the mains (placed in the street) to the house has small dimensions, typically 12 mm inside diameter or possibly even smaller. Some part of the building and network analyses is presented in P. K. Olsen(2008) SYSTEM PRINCIPLES Two different systems are included in the project, see figs. 1 and 2. The indirect system fig.1 has a heat exchanger for the heating system, allowing for a high network pressure. The direct system has a mixing valve and a shunt to control the flow temperature. This system has further two extra control valves to improve the cooling off. Both systems have a buffer for the domestic hot water system. Fig.1. Sketch of the indirect system. A tap flow is detected by a flow switch to start the the pump P2. The flow from the accumulator to the heat exchanger is controlled by a flow compensated thermostatically operated control valve. The 11th International Symposium on District Heating and Cooling, August 31 to September 2, 2008, Reykjavik, ICELAND Fig. 2 Sketch of the direct system. The water heater is similar to the indirect system, fig 1. That valves V1 and V2 makes it possible to improve the overall cooling of the system. Domestic hot water system is produced in a similar way as in the indirect system, fig. 1. DOMESTIC HOT WATER SYSTEM The system includes a storage tank and a heat exchanger. Heat is stored with district heating fluid as the medium. Domestic hot water is produced by a heat exchanger, supplied from the tank. A flow switch detects a water flow and starts the pump. The temperature supplied from the DH network is assumed always to have the same temperature (50 to 55 °C). The water temperature controller is flow compensated temperature type explained below. The tank is sized for the expected maximum daily draw off. The primary water flow is then adjusted to be constant. In the normal case with a smaller daily tapping, the primary flow is controlled on/off by a simple thermostat and a constant flow valve or just a constant hydraulic resistance. Using a higher primary flow reduces the necessary tank size and this may be analyzed including the properties of the network. The tank is filled from the top and the exchanger return is connected to the tank bottom to secure stratification. The tank inlets are designed to avoid mixing. Critical points in the system: • Stratification of the tank • Time delay in the taps caused by the pump starting • Standby loss of the system In this paper an overall analyses is given and the critical stratification is investigated. HOT WATER FOR THE HYDRONIC UNDER-FLOOR HEATING The indirect system is coupled in parallel to the domestic hot water system. The tap temperature is controlled by a (self acting or an electronic) control system. The direct system has a possibility to be supplied from the return water from the storage tank. In the case when the return from the under-floor heating system has a lower temperature than the return from the tank this flow is directed to the floor heating system to achieve an extra cooling off. The mixing circuit is controlled by a two way control valve in a traditional way. TRNSYS ANALYSIS OF THE SYSTEMS