GROUND SOURCE HEAT PUMPS DESIGNED FOR LOW-ENERGY, EARTH-SHELTERED ATRIUM BUILDING

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1 GROUND OURCE HEAT PUMP DEIGNED FOR LOW-ENERGY, EARTH-HELTERED ATRIUM BUILDING PAWŁOWKI Aleksander HEIM Dariusz HENEN Jan mgr inż. Aleksander Pawłowski, absolwent Wydziału Budownictwa, Architektury i Inżynierii Środowiska, Politechniki Łódzkiej, kierunek dyplomowania: Budownictwo Ekologiczne. łuchacz studium doktoranckiego. Tematyka zainteresowań: gospodarka energią w budownictwie, modelowanie i symulacja komputerowa wymiany masy i energii w budynku. olek.p@wp.pl mgr inż. Dariusz Heim, asystent w Katedrze Fizyki Budowli i Materiałów Budowlanych Politechniki Łódzkiej. Tematyka zainteresowań: modelowanie i komputerowa symulacja procesów transportu masy i energii w budynkach oraz procesów cieplno-wilgotnościowych zachodzących w jego elementach darkheim@p.lodz.pl prof. dr ir. Jan Hensen, professor of building performance simulation and research director of the Center for Building & ystems TNO TU/e, Technische Universiteit Eindhoven, Netherlands. GRUNTOWY WYMIENNIK CIEPŁA DO OGRZEWANIA I KLIMATYZACJI NIKOENERGETYCZNEGO, ATRIALNEGO BUDYNKU PODZIEMNEGO TREZCZENIE Główny cel stawiane budynkom podziemnym to redukcja strat ciepła w zimie oraz przegrzewania się budynku w lecie dzięki stabilniejszym warunkom termicznym występującym w gruncie. Dodatkowo roboty ziemne przeprowadzane na placu budowy podczas procesu budowlanego pozwalają na obniżenie kosztów instalacji takich jak poziome gruntowe wymienniki ciepła dla pompy ciepła. Z tego powodu korzystna jest integracja budynku podziemnego z systemem gruntowej pompy ciepła. Zasadniczym celem działań jest przeprowadzenie analizy projektowej budynku energooszczędnego oraz poziomego gruntowego wymiennika ciepła dla pompy ciepła ogrzewającej i chłodzącej dany obiekt. ymulacje komputerowe przeprowadzone zostały z zastosowaniem dwóch

2 typów warunków brzegowych: godzinowej bazy danych klimatycznych w skali roku dla okolic Warszawy, oraz miesięcznego rozkładu temperatur w występujących w gruncie w zależności od głębokości. Długości projektowanych poziomych wymienników ciepła wahają się między 180 a 300m w zależności od zapotrzebowania na ciepło w obiekcie oraz rodzaju gruntu, w którym wymiennik zostaje usytuowany. ABTRACT One of the main strategies in design of earth-sheltered buildings is to reduce heat losses in the winter and heat gains in the summer by using the relatively stable thermal conditions of the soil. Additionally, construction work carried out at the site allows a reduction of the installation costs for example for horizontal ground-source heat exchangers for heat pumps. Thus there are considerable advantages in integrating earth-sheltered buildings with horizontal ground-source heat pump systems. The principle objective of the current work is to conduct a design and performance analysis of an energy efficient building with a ground-source heat exchanger servicing the building's heat pump. The associated computer simulations used two types of temperature boundary conditions: - ambient hourly temperature over 1 year period for Warsaw and - monthly averages of depth-dependent soil temperatures. The design length of horizontal heat exchangers varies between 180m and 300m depending on peak heat load, type of surrounding soil and moisture content. For the purpose of this case study sandy, clayish and loamy type of soils with various moisture contents have been analyzed. A system's efficiency coefficient has been estimated based on a ground exchanger's fluid temperature and plant temperature. 1. TOPIC AND COPE OF THE PROJECT Due to a decrease of natural resources, growing degradation of a natural environment, raising fuel prices, political and economical instabilities there is growing ecological awareness focusing on: - optimisation of processes and material use - minimisation of losses - searching for and using of unconventional/natural sources of energy - recycling - carrying about natural environment There is an increasing pressure on scientists and design engineers to implement solutions reflecting the above trends in building design and architecture. Energy conservation presents the biggest challenge so far. It should be noted that global annual energy requirements represent only 1% of the sun energy reaching the earth with the remaining 99% of energy being wasted. For Polish climatic conditions about 80% of the total energy requirement of a house is for space heating and hot water (Fig. 1 with the highest loads occurring between December and February. Ironically - the solar energy availability in these months is the lowest.

3 80% 10% 8% % Heating DHW Electrical stuff Lightning Fig.1. Average energy consumption in a house. There are two basic issues to be addressed in a design of an energy efficient building: - how to minimise heat losses from the building - how to store solar energy until it is required To find solutions to this problem the dynamic thermal behaviour of an earth-sheltered building has been analysed. The moderate and stable temperatures beneath the earth surface offer opportunities for saving heating and cooling energy in most regions of the world. imply placing a house partially or completely underground does not automatically mean that energy will be saved. Energy-efficiency results from careful design during virtually all phases of the design process - site planning, building form and orientation, building envelope design, mechanical system design and construction details. Moreover, earth integration is rarely used as the only strategy for saving energy and is very often combined with other complementary approaches []. In this case study an active ecological heating and cooling system based on a ground-source heat pump system is proposed and analysed. This case study considers the dynamic thermal behaviour of the Earth heltered Arial Building. The building is located in Lodz (Poland at 5 o N latitude. The weather data which has been used is typical for that region. It is characterised by a continental temperate climate with during the summer average temperatures of 0C o (July, low overcast and good solar radiation and during the winter average temperatures of -5C o (January, significant overcast, low solar gain. The studied building has 70 m of heated floor area and 1,130 m 3 of heated volume. Fig.. Cross-section of the house. Fig.3. imulation model.

4 . PROBLEM DECRIPTION (building analysis and model creation. everal different construction options and some alternative solutions for energy management have been proposed during the course of the project. Technical considerations led to some modifications of the analysed building construction. The analysis process has been divided into two stages: - three different cases of building construction and - three different cases of energy control.1. Construction cases Case I Most of the building structure is located underground teel-concrete construction Centrally located open atrium provides good natural lighting Case II Most of the building structure is located underground teel-concrete construction Atrium covered with glass Case III The building structure is partly above (living room and hall and partly below ground level Centrally located open atrium provides good natural lighting.. Heating control cases A narrow-band temperatures -4C o, without daily temperature control system B wide-band temperature control 0-5C o, without daily temperature control system C wide-band temperature control 0-5C o, with daily temperature control system used 3. METHODOLOGY (running simulation. The EP-r energy simulation program has been used for analysing the heating and cooling demand of the building. EP-r (Environmental ystem Performance is an energy simulation system capable of modelling energy and fluid flows within combined building and plant systems when constrained to conform to control action. In this case, the heating and cooling systems are

5 modelled on a conceptual level; i.e. only the room processes are considered with idealized temperature controllers. The simulation results provide the amount of energy used per zone and per time step. The building has been divided into ten thermal zones (volume discretization to allow for more precise outcomes. The boundary conditions for the simulation include weather data and ground temperature profile. A third factor of the simulation was time discretization here we used one-hour time steps [4]. Ground temperature profile [*C] I II III IV V VI VII VIII IX X XI XII month Fig.4. Variation of temperature distribution in the ground over the year [6]. 4. REULT PREENTATION (simulation results analysis The simulation results include hourly data for heating and cooling energy demand for each of the analyzed cases. Annual heating demand [kwh] IA IB IC IIC IIIC 0 Jan Feb Mar Apr May Jun Jul Aug ep Oct Nov Dec Fig.5. Heating demand for the analysed cases.

6 Annual cooling demand [kwh] Jan Feb Mar Apr May Jun Jul Aug ep Oct Nov Dec I-A I-B I-C II-C III-C Fig.6. Cooling demand for the analysed cases. Figure 5 and figure 6 show graphical results of the heating and cooling demand variation over a year for the analysed cases. In conclusion - case II C provides the most energy efficient solution. 5. HEAT EXCHANGER IZING Due to the challenge of appropriate system components sizing, a process of building a model of heat transfer between the ground and a heat exchanger connected to a heat pump used for heating or cooling of a building is considered to be really difficult. In theory, such a model should be able to predict the influence of soil thermal properties, (density, moisture content, temperature distribution, heat exchanger material, pipe diameter, heat exchanger fluid properties, fluid film resistance, and thermal contact resistance between the pipe and the soil on the operation of the heat exchanger. Furthermore, in order to forecast accurately the ground heat pump performance, it is necessary to estimate the real effects of equipment cycling, seasonal earth temperature changes, and heat exchanger surface temperatures below freezing. Nevertheless, it appears to be a quite sophisticated mathematical challenge to find a satisfactory solution to all these factors in one method. For that reason, the ground heat transfer process is generally oversimplified only for the purpose of obtaining forms that are mathematically tractable. This can eventually lead to overestimating heat exchanger size what can turn out to be extremely uneconomical or to underestimating which, in turn, means that the heat exchanger is not able to transfer a sufficient amount of energy. As a consequence, the computer program called G000TM (first released in 1995 in Canada was introduced.

7 It allows sizing of ground heat exchangers for both commercial and residential systems. ince G000TM is able to predict accurately the necessary heat exchanger size, the engineer may reduce installation costs simply by implementing appropriate changes to the heat exchanger design. It needs to be pointed out that G000TM uses as its basis Kelvin line-source theory increased by empirical data from field experiments [1, 3]. 6. MATHEMATICAL MODEL To illustrate the form and complexity of the equations for solving earth heat transfer problems we introduce two of the design and prediction methods [1]. The International Ground ource Heat Pump Association Method oil resistance of single vertical heat exchanger I( X r o Rs ( X = πk s where X ro r o /{(α,t 1/ } and I(X ro is exponential integral r o outside pipe radius α s diffusivity of the soil k s thermal conductivity of soil t time of the year oil resistance of single horizontal heat exchanger R s I( X r I( X BD o = πk where I( X I( X ro (πk BD (πk s - resistance of soil surrounding a single pipe of - resistance of outside radius r an imaginary pipe located at an equal distance above the surface ofthe earth as the buried pipe, where BD is the burial depth o T ( X, t = T M π A exp X 365α 1 π X cos t to x πα 1

8 Lowest or minimum annual soil temperature at depth X s T L = T M π A exp X 365α 1 Highest or maximum annual soil temperature at depth X s T H = T M π + A exp X 365α 1 Length of trench required for heat exchanger size in heating and cooling period. L H COPH CAPH COP H = 1 COPC 1 ( R + R F CAPC ( Rp + R FC ( T T L MIN p H L C = + COPC ( T T MAX H 7. TARTING THE HEAT EXCHANGER IMULATION The required inputs to start any simulation with the G000 TM are: Heat exchanger configuration refers to the how will be placed in the ground. The pipes can be positioned vertically in drilled boreholes or horizontally in trenches. Ground temperature properties near surface ground temperature cycles around over a year. These variations tend to disappear at lower depths, where the ground remains at its mean temperature throughout the year in anticipated localization (Fig. 4.. Ground layer description - different soils and rocks will have different thermal properties, which will impact on a required heat exchanger size. G000 TM allows the user to select or specify different ground materials and different ground properties specification for summer and winter to account for different moisture levels during heating and cooling seasons. Heat Pump ystem Design - define characteristics of the heat pump system. Ground Load Inputs like in (Table 1. Jan Feb Mar Apr May Jun Jul Aug ep Oct Nov Dec Total: Heating Cooling Table 1. Input data - monthly energy demand in Watt s for the best performing building case (C-II. The vertical models in this program cannot model ground freezing. Consequently, some accuracy will be lost if the minimum entering water temperature is allowed to drop below freezing.

9 The bar graph (Fig.7. shows the simulation results relating to the length of the horizontal heat exchanger, for case II-C, for two different kinds of ground and various humidity of them. In all examples the exchanger is laid horizontally in a trench m below the ground surface. [m] ,7 The trench lenght required for heating and cooling 60,1 40,1 183, Fig.7.Variation of trench length required for heating and cooling. where 1 silty clay/clay saturation -10% silty clay/clay saturation 10-0% 3 silty loam saturation -10% 4 silty loam saturation 10-0% 1 97,7m 60,1m ,1m 183,1m Fig.8. Monthly fluid temperatures (G000 output.

10 The graphs (Fig.8. shows the simulation results relating to the monthly fluid temperatures entering the heat pump for both the cooling and heating design lengths of considered heat exchangers. 8. CONCLUION Widening of the controlled temperature range results in about 16.5% reduction of heating and almost 17% reduction of cooling energy requirements. Applied daily temperature control system resulted in 10% reduction of heating and 15% reduction of cooling requirements. Earth-sheltered house proved to be 13% to 17% more energy efficient when compared with the above ground construction. Different grounds' properties and the levels of moisture during heating and cooling seasons have got huge impact on the length of the heat exchanger. Different soil types, their properties and moisture levels can substantially influence the designed length of the heat exchanger. In the analysed case the heat exchanger length in different soil types can be reduced by more than 60% (Fig REFERENCE [1] CANE P., FORGA D.A.: Modelling of ground-source heat pump performance, AHRAE Transactions, ymposia NY R.L.D., [] CARMONDY J., terling R.: Earth sheltered housing design, New York [3] CHWIEDUK D.: łoneczne i gruntowe systemy grzewcze. Zagadnienia stymulacji funkcjonowania i wydajności cieplnej. Warszawa [4] CLARKE J.A., Energy simulation in building design, nd edition, Butterworth Heinemann, Oxford, 001. [5] PAWŁOWKI A.: Atrialny Budynek Pasywny - Analiza cieplna dynamicznego zachowania się budynku wraz z gruntowym wymiennikiem ciepła zaprojektowanym dla celów grzewczych i klimatyzacyjnych.; praca magisterska na Wydziale Budownictwa, Architektury i Inż.. Środowiska, Politechnika Łódzka, 00. [6] RUBIK M.: Pompy ciepła. Poradnik Ośrodek Informacji "Technika instalacyjna w budownictwie", Warszawa 1999r.