LOW-COST LOW-TEMPERATURE HEATING WITH HEAT PUMP

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1 LOW-COST LOW-TEMPERATURE HEATING WITH HEAT PUMP THOMAS AFJEI Information Center for Electricity Applications INFEL Lagerstrasse 1, CH-8021 Zurich, Switzerland Tel , Fax , . 1 SUMMARY Future low-energy dwellings have an annual heat demand of under 200 MJ/m 2 a yielding a % share for the hot water heating. Simulations with TRNSYS showed that it is possible to connect the heat pump directly to the distribution system without buffer storage. Operating interruptions of two successive hours are not perceptible in the room due to the inertia of the floor heating. During periods of strong solar radiation, especially in light weight building constructions a slightly higher room-temperature must be accepted. The system with the highest SPF is a brine/water heat pump with separate heat pump water heater, yielding a SPF of 4.1 for space heating and 3.2 with hot water production. 2 INTRODUCTION For future low-energy dwellings with an energy factor of under 200MJ/m 2 a there is an urgent requirement for a low-cost heat pump heating system with a high annual energy efficiency. Such a heating system must be competitive in the new building sector with conventional oilfired heating. Proposed as a solution for the heat distribution is a low-temperature heating system with a feed temperature of <30 C which has a good self-regulating effect without additional controlling elements. The heat pump is connected directly to the heat distribution system without buffer storage and mixing valves. It is to be provided with an intelligent control, integrated by the manufacturer in the heat pump. The simple construction will result in low installation costs. The interactions between low-temperature heating and building are of great significance. Applied as a criterion for the physiological comfort is the predicted percentage of dissatisfied (PPD) according to O. Fanger. Factors to be determined by simulation are: How should the low temperature heating be structured? What requirements must the building satisfy? What control range is necessary for the heat pump? What effect will disturbance variables have on comfort, i.e. solar radiation, temperature change, heat sources etc.? What utility off-periods can be tolerated? What will be the annual energy efficiency of the heat production? The information and experience gained serves to clarify feasibility and problem analysis of such heating systems. Of interest are also the achievable seasonal performance factor and the level of the market opportunities of such a system for the new building and renovation market, incorporating hot water heating system. I.I.F.-I.I.R. - Commissions E2, with E1 and B2 - Linz (Austria)

2 3 SELECTION OF A REFERENCE BUILDING Selected for this project was the NOAH-System house of the Swiss firm A. Piatti AG. Basic reasons for the choice were the wide market acceptance of this building (100 have already been erected) and the circumstances of a uniform small thermal energy requirement over the ground plan and no excessively large glazed surfaces. Also important for this project was the fact that a good data basis was available for engineering and costs. Already included in the requirements in Fig. 1 for the DIANE Eco-construction, Vision 2050 Canton ZH and SIA 2020 is a ventilation system. These were shown as extra with the Requirements for the HP-Project and the NOAH-House but were not used for cost reasons. Fig. 2 and Fig. 3 show a photo and the energy flux diagram of the NOAH house. *+, "# $% &' ( ) Fig. 1: Thermal energy requirements for different organisations and for this project (HP- Project); possible savings potential of a mechanical ventilation system (shown in light) Fig. 2: NOAH low-energy house (Photo) The most important characteristics are: Thermal energy requirement (Qh, Swiss Standard SIA380 [1]) 174 MJ/m 2 a Heat output (-11 C, without hot water) 3.1 kw average specific heat output 22.0 W/m 2 assumed air change (-11 C) 0.3 h -1 Roof (U-value) 0.22 W/m 2 K External walls (U-value) 0.29 W/m 2 K Windows (U-value) 1.1 W/m 2 K

3 " # " $ % & '()*+,- Fig. 3: Energy flowchart of the utilised low-energy house NOAH 4 RESULTS OF TRNSYS SIMULATION Detailed computer simulations using TRNSYS [2] are presented, analysing the whole system comprising heat source, heat pump, regulation and controls, heat distribution system and the building itself. The selected systems for prototype testing were as depicted in Fig. 4: Small sized air/water heat pump with an auxiliary electric resistance heater, Larger stand-alone air/water heat pump and Brine/water heat pump with ground heat exchanger Fig. 4: Schematics of the heat pump systems for prototype testing

4 The simulations were carried out with data taken from components available on the market. The results showed that for low energy houses with floor heating systems, it is possible to connect the heat pump directly to the distribution system. Mixing-valve, buffer storage and controls are no longer necessary. During periods of strong solar radiation, a slightly higher room-temperature must be accepted, especially on the upper floors and with lighter weight buildings. The PPD-Value (predicted percentage of dissatisfied) for lighter weight construction is 6.7 % and for massive construction 5.8 %. It should however be mentioned that these figures consider only buildings with small window ratio. For larger window ratios and for solar architecture, an increase in weight by adding mass (the fluctuation of room temperature can be halved by changing to heavier weight construction) and if necessary further measures, such as solar sensors for the regulation of the heating system, are necessary. For comparison purposes, numerous combinations of brine/water and air/water heat pumps with separate and integral hot water heaters were calculated in the study. The separate heaters are heated either electrically or by heat pump. The integral heater is heated from the heating system via heat exchanger. As expected, the ground-coupled heat pump with separate heatpump water heater with heat recovery from exhaust air achieved the highest seasonal performance factor (SPF), amounting to 4.1 for space heating only and 3.2 with additional hot water production. The calculations were done with data from average state of the art heat pumps. If more advanced units are assumed, SPF values of 5.1 or 4.0, respectively, can be achieved. This also applies in the case of utility off-periods of 4 hours per day. The thermal behaviour was calculated using the TRNSYS simulation program. The program simulates ground heat exchanger (approximated by a load profile), heat pump, auxiliary electric heater, controls and floor heating. The simulation is based on hourly values of outside air temperature, on direct (horizontal) and diffuse radiation. Meteorological data from the Test Reference Year (TRY) for Zurich-Kloten was used. In comparison to actual measurements, simulations have the advantage that the energy for the different variants may be evaluated independently of user behaviour. Hot water production, which can amount to as much as a third of the heat demand of a low energy house, is provided by heat pumps. For this, the variants of a separate heat pump water heater and (for the ground coupled heat pump) an integral water heater were investigated. The temperature of the water was kept constant at 45 C, and to avoid legionella, heated up daily to 55 C. Low temperature floor heating systems can be realised with a specific heat demand as low as 30 to 40 W/m 2. Thanks to the low heat flux, supply temperatures are only a few Kelvin above room temperature (design values for an outside temperature of -11 C: supply 30 C, return 25 C). The self-regulation of low temperature floor heating turned out to be very effective - even for parquet flooring. It is therefore sufficient to have a two point control of the return temperature in place of the usual supply temperature control. Inclusion of the usual utility off-periods (in the calculation from to 07.00, to and to 24.00), with the advantage of reduced electricity rates, did not have an effect on comfort. An increase in heating rate is however necessary due to the shut off, requiring a higher supply temperature. This results in a reduction in seasonal energy efficiency of 5 %. Night setback is not considered since the slightly reduced heat loss during the night is not sufficient to compensate for the reduction in heat pump efficiency due to the higher supply temperature.

5 5 ECONOMY In the marketplace, this simple low temperature heat pump heating system has to compete mainly with small oil-fired boilers. In areas where inexpensive natural gas is available, the chances of selling are less favourable. A comprehensive cost calculation produced an encouraging result: the proposed low temperature heating system has in fact about the same costs as a small oil-fired boiler (see Fig. 5). " " " # $ $ % & "& Fig. 5: Annual costs for heating and hot water production in US$

6 6 INFLUENCE OF ELECTRICITY PRODUCTION The fuel requirements can be reduced to one-half compared with a an oil-fired boiler when the electricity for the heat pump is produced by a cogeneration plant or a modern combined cycle plant (see Fig. 6). " " " # $ $ % & "& Fig. 6: Annual energy efficiency for different heat pump systems based on primary energy 7 CONCLUSIONS Interruption due to utility off-periods Operating interruptions of a few successive hours are not percetible in the room with underfloor heating. Therefore there are no objections for comfort reasons against off-periods. However a reduction in running time does have a disadvantageous effect on the seasonal performance factor. Instead of a power supply to the building over 24 h, the same amount of energy has now to be supplied in a shorter time. At full load this can only be dealt with by an increase in the heat pump capacity and the system temperatures. An interruption of 4 h per day requires a 20 % increase in heat pump capacity. This impairs the seasonal performance factor by 5 %.

7 Integration of the heat pump The storage capability of the floor is quite sufficient to operate the heat pump heating without buffer storage. The heat pump switches off no more than three to four times per day. Accordingly a buffer storage would only lead to additional heat losses, render the regulating concept more complex and raise investment costs. It is necessary to switch the heat pump on and off according to the return and not the supply temperature. Only in this way can the storage capability of the heat distribution system and the floor be utilised. With supply temperature control, the heat pump would continuously switch on and off (cyclic operation). System selection The most energy-optimal system is the brine/water heat pump with separate water heater heat pump. An increase in energy consumption of 10 % must be accepted in the case of hot water heating from the heating heat pump. The seasonal performance factor of the space heating system is 18 % better when by means of a ground heat exchanger, ground heat is used as a heat source instead of air. If the A/W heat pump is dimensioned 30 % smaller and the capacity loss covered by an electrical resistance heating, the cost of electricity for the heating only increases by 4 %. A precondition however is the correct ON and OFF switching of the auxiliary heating, otherwise the extra consumption is considerably higher. Cost considerations With annual costs as a criterion, the monovalently dimensioned air/water heat pump with separate electric water heater is the lowest. The extra costs of the exhaust air heat pump for the hot water heating compared to the electric water heater solution with the brine/water and air/water variants are not amortizable by the lower energy costs. On the other hand the exhaust air heat pump has the advantage that a controlled air change takes place, thus improving the air quality. Further information can be taken from the the final report Kostengünstige Niedrigtemperaturheizung mit Wärmepumpe [3]. 8 ACKNOWLEDGEMENTS This project was commissioned by the Swiss Federal Office of Energy (SFOE). A project team was assembled from specialists from diverse sectors, comprising: Information Centre for Electricity Applications (Project Management) Basler + Hofmann AG / DIANE Eco-construction (Building) Bircher + Keller AG (Underfloor heating) Swiss Federal Institute of Technology, Zurich / Institute for Measurement and Control (State controller) Central Suisse Engineering College, Lucerne ZTL (TRNSYS Simulation) The project was supported by an accompanying group in which was represented the Swiss Federal Institute of Technology SFOE, an architect, a building contractor for low-energy dwellings, also house installation specialists and a heat pump producer.

8 9 REFERENCES [1] SIA 380/1; Energie im Hochbau; Edition 1988; Reprint 7/1993; Zurich, CH. [2] TRNSYS 14.2: University of Wisconsin, Solar Energy Laboratory, 1996, Madison WI, USA. [3] INFEL: Kostengünstige Niedrigtemperaturheizung mit Wärmepumpe, Schlussbericht Phase 1, BEW-Projekt 55701, ENET (order no ; address: ENET, Administration und Versand, Postfach 130, CH-3000 Bern 16, Schweiz, Fax +41-(0) ), CH. RESUME Les futures habitations à basse consommation d'énergie auront une demande de chaleur inférieure à 200 MJ/m² a, comprenant environ 30 à 40% pour la préparation d'eau chaude. Des simulations avec TRNSYS ont montré qu'il est possible de connecter la pompe à chaleur directement au système de distribution sans stockage en tampon. Des interruptions d'exploitation de deux heures d'affilée ne sont pas perceptibles dans les chambres en raison de l'inertie du chauffage par le sol. Pendant les périodes de forte radiation solaire, spécialement dans les constructions légères, une température ambiante légèrement supérieure doit être acceptée. Le système offrant le meilleur coefficient de performance saisonnier est une pompe à chaleur saumure / eau avec un chauffe-eau à pompe à chaleur séparée, offrant un coefficient de performance saisonnier de 4.1 pour le chauffage de l'habitation et de 3.2 avec la production d'eau chaude.