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Renewable Energy 34 (2009) 2935–2939

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Renewable Energy

journal homepage: www.elsevier .com/locate/renene

Technical Note

An investigation into the heat consumption in a low-energy building

K. Wojdyga*

The Warsaw University of Technology, 20 Nowowiejska St., 02668 Warsaw, Poland

a r t i c l e i n f o

Article history:Received 24 October 2008Accepted 1 April 2009Available online 1 May 2009

Keywords:Low-energy buildingSolar collectorsHeat consumption

* Tel.: þ48 22 2345310; fax: þ48 22 8258964.E-mail address: krzysztof.wojdyga@is.pw.edu.pl

0960-1481/$ – see front matter � 2009 Elsevier Ltd.doi:10.1016/j.renene.2009.04.001

a b s t r a c t

The results of five-year study of the heat consumption in a single-storey terraced low-energy residenceare presented. The house is part of a forty building Warsaw housing estate Wilanow–Zawady.

� 2009 Elsevier Ltd. All rights reserved.

1. Introduction

The international obligations undertaken by Poland necessitateits taking steps toward lowering its emission of greenhouse gasesand rationalising its energy use in order to comply with current EUdirectives. The implementation of these directives should signifi-cantly increase the effectiveness of the country’s energy productionand energy use. Two factors influence heat demand in Poland: anincrease in industrial production and increased living standardsthat will cause an increase in energy demand and the accompa-nying rationalisation of energy use and a corresponding significantreduction in demand for energy.

The volume of heat use for heating purposes directly dependson the standard of the building. This can be characterised by anindex for seasonal demand for heat in order to heat 1 m2 of heatedarea during a standard heating season – the index is described as Es

[kWh/m2a]. For instance, buildings built in the years 1979–88 werecharacterised by the value of an index for seasonal demand ata level of 340 kWh/m2a, in comparison with buildings built before1970 (E¼ 170 kWh/m2a), and for buildings built after 1996 (index Emaximally amounted to 160 kWh/m2a). There are also so-calledlow-energy buildings whose demand for heat for heating purposesreaches a level of 30–40 kWh/m2a, and occasionally 10–15 kWh/m2a, in the case of passive houses.

It needs to be stressed that over the last ten years residentialbuildings have been subjected to intensive thermomodernisationprocesses. The economically justified revitalisation of a buildingallowed to reduce its index for seasonal heat demand to a level of

All rights reserved.

110–140 kWh/m2a, corresponds to the reduction of the mean valueof this index to a level of 160 kWh/m2a.

2. Heating characteristic of the low-energy building

Since the mid-1990s the so-called low-energy buildings havebeen built (also known as energy saving) and there are typicallysingle-family, terraced houses. In 2001, in one of Warsaw’s resi-dential estates a building complex consisting of forty terracedhouses was opened. The buildings differ among themselves as far astheir utility surface and room layout is concerned. Each of theterraced houses is located on a 383 m2 plot of land and has a south-north orientation with a 30� diversion to the east. The entrance tothe building is located on the northern side, while the garden andterrace are found on the southern side. The total utility size of thehouse is 242 m2. The garage constitutes an integral part of thehouse and is connected with the living quarters by means of a non-heated vestibule. On the ground floor there is a corridor, kitchen,bathroom, study, and living room. The size of the living room isincreased by an apse. The ground floor is connected with the firstfloor by an open staircase. Large-paned window in an apse anda roof skylight are the main source of natural light. Above the firstfloor there is a utility attic.

The building was intended to be low-energy from the earliestdesign stages. All heat insulation conformed to standards above theprevailing norm (namely PN-91/B-02020). The outer walls weremade with hollow type F 30 cm thick bricks. Additionally, the wallswere insulated with a layer of foamed 10 cm thick polystyrene. Theheat-transfer coefficient U equalled 0.18 W/m2K, while the stan-dards required that the coefficient U should be no higher than0.55 W/m2K. The heat insulation of the roof was also increased by50% so that U¼ 0.2 W/m2K, when the required normative value was

Table 1Building’s energy factors.

Standard building according to PN-91/B-02020 Low-energy building

Heat-transfer coefficient U – external walls 0.55 W/m2K 0.18 W/m2KHeat-transfer coefficient U – roof 0.3 W/m2K 0.2 W/m2KHeat resistance R – sub-floor 1.5 m2K/W 2.6 m2K/WHeat-transfer coefficient U – windows 2.9 W/m2K 1.1 W/m2KHeat power demand for �20 �C 12.2 kW 7 kWIndex for seasonal heat demand in relation to utility surface 95.9 kWh/m2a 30.6 kWh/m2aIndex for seasonal heat demand in relation to volume of house 17.4 W/m3 10 W/m3

Fig. 1. Photos of low-energy building in Wilanow–Zawady.

K. Wojdyga / Renewable Energy 34 (2009) 2935–29392936

0.3 W/m2K. The heat insulation of the floor was also increasedabove the prevailing standard by nears of 10 cm of foamed poly-styrene. Furthermore the building’s foundations were insulatedabove the standard. The window woodwork was also of a higherstandard where a single-frame window with a thermoinsulatingpane combined with an argon resulting in a coefficient ofU¼ 1.1 W/m2K. Moreover the windows and the balcony door werecovered with external aluminium blinds (filled with polyurethanefoam). By closing the shades during low temperatures, a significantreduction of heat loss through the windows was achieved. It hasbeen observed that the temperature in the space between thewindow and the blinds is higher by 10 �C than the temperatureoutside.

The building is heated by means of electric heaters located ineach room. These heaters are controlled and regulated by a systemwhich enables random adjustment of temperature and which takesinto account the presence or absence of the occupants, raising orlowering accordingly the temperature. The installation ofmechanical ventilation with heat recovery from the removed airconstitutes an additional heating system. This installation consti-tutes the main heat source in the transition period of the heatingseason. Fresh air is delivered from outside through a wall air boxand carried to the economiser. In the plate cross-flow heatexchanger cold air from outside and warm air from the rooms passeach other. The cold air is heated by the warm air but does not mix

Table 2Electricity consumption and the number of degree-days in heating seasons.

Heating season Electricity consumption [kWh] Number of degree-days

2002/2003 18,800 40962003/2004 14,600 37022004/2005 13,850 35442005/2006 15,400 39552006/2007 12,100 3016

with it. Because of this a 60–70% heat return is possible. Moreover,the fresh air after having been heated in the exchanger is addi-tionally heated, with a 2 kW electric heater, up to the temperatureprogrammed in the controller. Air from outside, once heated, ispumped through the ventilation channels and regulation valvesand is blown into the rooms. The used air is extracted through thevalves and the channel system from the kitchen and bathroomsbefore being removed into the atmosphere by means of an air ductinto the chimney. The flow of air passing through the recuperator isregulated and changes within a range of 90–400 m3/h. Taking intoconsideration the cubic capacity of the building (700 m3), themultiplication factor of air exchange can vary from 0.13 to 0.6 of anexchange per hour.

When the temperature outside is lower the fireplace, with a castiron input and a closed emission circuit, is activated reachinga maximum heating power of 5 kW. The hot air is circulatedthroughout the whole house.

The building is equipped with hot and cold water installations.In winter, the main source preparing hot tap water is a 300 lcontainer, in which the water is heated with an electric 2 kWheater. Throughout the year the warm water installation is sup-ported by a system of vacuum solar collectors. These panels satisfy90% of demand for warm water between April and October.

All houses have been constructed with the aim of meeting low-energy standards. The basic figures for this type of terraced buildingare:

� Volume of the heated part of the house – 700 m3.� Building’s utility surface – 242 m2.� Building’s shape coefficient A/V¼ 0.68.� Number of floors – two floorsþ attic.

Table 1 presents the main energy factors for a standard buildingand for a low-energy building.

Building is shown in Fig. 1.

Table 3Electricity consumption for various purposes.

Heating season Energy – total [kWh] Hot tape water [kWh] Home equipment & appliances [kWh] Garage driver heater [kWh] Heating [kWh]

2002/2003 18,800 2590 3640 2020 10,5502003/2004 14,600 2790 3640 1200 69702004/2005 13,850 3100 2850 1200 67002005/2006 15,400 2580 2900 1200 87202006/2007 12,100 1900 3200 1000 6000

K. Wojdyga / Renewable Energy 34 (2009) 2935–2939 2937

The heating of the terraced house began in October 2001,though regular residence of the building only started in May 2002.The monitoring of energy use for the building’s heating purposes aswell as for the preparation of hot tap water commenced togetherwith the heating of the building. However, reliable data aboutenergy use over a period of regular residence of the building withfour people living there, began with the heating season 2002/2003.Also, in this period a monitoring system of electric energy use wasinitiated for the purposes of hot water. It should be pointed out thatthe houses adjacent to the monitored house were not heated in the2002/2003 heating season, while in the 2003/2004 heating seasonthey were heated only up to a duty temperature in the region of10–15 �C. Due to this, heat use in the monitored house increased.

3. Investigation of heat consumption in the building

The calculation of electric energy use was carried out for fiveconsecutive heating seasons. It was assumed that a heating seasonstarted on 1st October and ended on 30th April. For the sameperiod, the number of degree-days was calculated for a heatingseason defined in the above manner. The total electricityconsumption presented in Table 2 concerns use over the entirebalance year from 1st October to the end of September the nextyear.

Total electricity consumption covers use for the followingpurposes:

� heating the building,� hot tap water preparation,� lighting and electric home equipment and appliances,� heating the stairs and garage drives.

Table 3 shows the division of annual heat use for the above-mentioned purposes.

Energy consumption for the heating of the building is closelyconnected to the heating season (defined by the number of degree-days). Table 4, besides energy use for heating purposes, also showsindex E of the seasonal energy demand corresponding to 1 m2 ofthe surface. In the next column the same index has been presentedcorresponding to the standard value of degree-days. For Warsaw,the standard number of degree-days is 3884. For this value index Ehas been calculated at 30.6 kWh/m2a.

The mean value of the corrected index E of the seasonal energydemand for four consecutive heating seasons (2003–2007) was31.9 kWh/m2a and is slightly higher than the value for a standardheating season. Thus, the results of a few years of measurements

Table 4List of seasonal energy demand indices.

Heating season Heating use [kWh]

2002/2003 10,5502003/2004 69702004/2005 67002005/2006 87202006/2007 6000

have confirmed that the building satisfies the criteria of a low-energy building.

The second indicator characterising the heating qualities of thebuilding is maximum power demand. For standard temperature�20 �C, the heat power required for heating purposes is 7.03 kW.The maximum registered 24-hour energy use in the period 2001–2004 was 154 kWh. Subtracting from this value 20 kWh (used forother purposes), we have 134 kWh. This gives us a heat powerindex of 5.6 kW. The 24-hour energy use is shown in Fig. 2.

4. Solar domestic hot water system

The installation under consideration serves for the preparationof hot tap water using electricity and renewable energy by means ofsolar collectors. It was installed in a single-family house inhabitedby four people. A 300 l tank is used to heat the tap water. The tank isinsulated with polyurethane foam ca. 10 cm thick. The main sourceof heat in the tank is a 2 kW electric heater. The tank also containsan exchanger fed by a heating liquid (glycol) heated in the battery oftwo solar collectors installed on the roof. Evacuated tube solarcollectors consist of a absorber plate and heat-transfer tubes. Ineach collector tube there is a U shaped copper pipe. The absorberplate is coated with a high efficiency selective coating. The totalabsorption factor is higher than 90% and the emissive coefficient isless than 9%. In a collector the copper pipe and absorber plate arehermetically sealed within an evacuated glass tube. The pressure inthe evacuated tube is about 10�5 mbar. The active surface of thetwo sets of vacuum pipe collectors jointly amounts to 3.4 m2. Anelectronic digital regulator steers the work of the installation of thesolar collectors (Fig. 3).

The aim of the analysis in question, investigating the functioningof solar installation for heating hot tap water, was to estimate heatsurpluses from the solar energy used. It required estimating thequantity of solar energy and electricity which was used for heatingwater. In the present research the following quantities weremeasured and registered:

� electricity consumption for preparing hot water in the tank,Eel [kWh],� volume of tap water feeding the tank, V [m3]� total irradiance on the plane of the collectors, Ib [W/m2],� temperature of cold water supplying the tank, tc [�C];� temperature of water in the tank at 2/3 of its height, th [�C];� temperature of the heated liquid from the collector, t1 [�C];� temperature of the heated liquid as the input to the tank, t20 [�C].

Actual index [kWh/m2a] Corrected index [kWh/m2a]

43.6 41.328.8 30.227.7 30.336.0 35.424.8 31.9

020406080

100120140160180

2001

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24-hour electricity consumption from Nov. 2001 to May 2004

Fig. 2. 24-hour electricity consumption from Nov. 2001 to May 2004.

K. Wojdyga / Renewable Energy 34 (2009) 2935–29392938

The measurement of electricity consumption for preparing hotwater Eel was carried out by means of an electricity metre attachedto the heater in the tank. For measuring the volume of the heatedwater Vcw a vane-wheel water metre was used. All temperaturemeasurements were carried out with the use of specialist recorders(so-called dataloggers). The measurements taken by each of thetemperature sensors were automatically logged in their memoryevery 5 min. The measurement of solar irradiance intensity wasdone with the use of a solarimetre located on the roof, on the sameroof surface as the solar collectors. As a consequence we were ableto measure the solar irradiance on the surface oriented in the sameway as the surface of the collectors. The solarimetre was scaled involts. The constant of the solarimetre amounted to 4.43�10�6 [V/W/m2]. The readout and registration of the results were carried outwith the use of a universal recorder connected with a computer bymeans of a serial port.

Fig. 4 presents the mean values of the intensity of total solarirradiance and the corresponding daily sums of radiation. The meanvalues were as follows: for the last eight days of April the meanintensity of total irradiance amounted to 402.07 W/m2, while forMay and June 401.11 W/m2 and 444.69 W/m2, respectively. Thesums of daily radiation for the last eight days of April amounted to

Vz.w.

Eel

tzw

ti

tcwu

t2

c.w.u.

z.w.

te

t1

I

Fig. 3. Diagram of solar installation preparing hot tap water.

37.6 kWh/m2 and for May and June 129.5 kWh/m2 and 130.8 kWh/m2, respectively.

Over 17 days in April, hot water use was measured at 4.28 m3, inMay it was 7.02 m3, while in June 5.83 m3. The mean value of 24-hour use over the entire analysed period equalled 0.225 m3. Thetotal amount of prepared hot water in that period was 17.128 m3.The use of electricity needed for heating water over the sameperiod at low solar irradiance intensity were: 154.9 kWh,178.7 kWh, 76.4 kWh, respectively. Over the entire analysed period,the mean temperature of hot tap water was 49.57 �C.

By comparing the value of energy theoretically needed forpreparing warm water over a given period (e.g. a 24-hour timeperiod) with the electricity actually used for this purpose, it waspossible to determine the energy savings DE generated by the solarinstallation. Energy saving corresponds to ‘‘net’’ solar energy, i.e.the solar energy actually used for heating warm water. The value ofnet solar energy corresponds to the sum of solar radiation incidenton the surface of the two collectors less the value of energy lossesfrom pipes, from the resistance of the heat exchanger transfer in theheater, from the efficiency of the process of photothermic conver-sion of solar radiation in collectors as well as from heat losses of thecollectors themselves. The summary values of energy theoreticallyneeded for preparing warm water for consecutive months of theperiod analysed amounted to: 197.7 kWh for 17 days of April,293.1 kWh for May and 217.9 kWh for 28 days of June. Energysavings for the above-mentioned periods amounted to 42.8 kWh,114.5 kWh and 141.5 kWh, respectively. A general share of solarradiation for the above-mentioned periods was 33%, 26% and 32%,respectively.

For comparison, Fig. 5 shows a graph representing the 24-hourelectricity used for the purposes of preparing hot tape water.

The analysis of the work of the solar collectors heating hotwater covered the period from 14th April to 28th June 2003. Theanalysis made it possible to estimate the value of energy whichwould have been needed for preparing hot tape water in theinstallation without solar collectors, and compare it with theamount of electric energy actually used for heating hot water. Dueto this, a calculation of the surpluses resulting from the imple-mentation of solar collectors was possible. The total amount ofprepared hot water Vtot in the entire period analysed amounted to17.128 m3. The mean value of temperature for the entiremeasurement period during which water was heated reached thevalue of 49.57 �C. The total energy Et,tot theoretically needed forheating this amount of water amounted to 708.8 kWh. The amountof electricity consumption Et,tot reached the summary value of410.0 kWh, which constituted 57.8% of total theoretical energyEt,tot. During the measurement period, use of solar energy forheating propose was 298.8 kWh, which is about 42% of the totalenergy needed to heat hot tap water. Over the period of 76 days,during which the investigation was carried out, the sum total solarradiation Es,tot across the surfaces of the 2 collectors with a jointsize of 3.4 m2 was 1.013.0 kWh.

5. Summary

The results arrived at support arguments in favour of the use ofsolar collectors co-operating with hot tap water preparing instal-lations. A reduction in the cost of solar installations couldcontribute to a remarkable increase in their numbers in newly-builtresidential buildings. Heat surpluses from solar energy for waterheating are significantly high. During measurements the share ofsolar energy in heated water was about 42% and solar incident in30% was change to heat.

As can be seen from the above analyses, the low-energy buildingunder consideration meets expectations and confirms the theoretic

Mean values of the intensity of irradiance and daily sums of radiation

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04-22 04-27 05-02 05-07 05-12 05-17 05-22 05-27 06-01 06-06 06-11 06-16 06-21 06-26

Data

Es [

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700I

[W/m

2]Es

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Fig. 4. Mean values of the intensity of irradiance Ib and daily sums of radiation Es.

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14-04

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data

24-hour electricity consumption in the preparation of hot tap water

Fig. 5. The 24-hour electricity consumption for preparing hot tape water.

K. Wojdyga / Renewable Energy 34 (2009) 2935–2939 2939

calculations of heat power demand as well as those of heat use forheating demands. In the same weather conditions with standard

factor of degree-days the low-energy building uses 3 time lessenergy (30.6 kWh/m2a) then a building designed in accordancewith Polish standard (95.9 kWh/m2a). The definitions of passiveenergy house or low-energy house must depend on weatherconditions of the country concerned. In countries with a warmerclimate it is much easier to fulfil the criteria of passive and low-energy houses. The construction of low-energy buildings with verygood heat insulation and additionally equipped with mechanicalventilation installations with heat recovery, solar installation forpreparing hot water and other known energy-saving solutions doesnot require a noticeable increase in financial outlay at theconstruction stage. The estimated increase in construction costs is7–10% in comparison with a standard building. These additionalinvestment costs will pay for themselves very soon in the form ofenergy saved, the price of which continues to grow. Spreading theidea of the construction of low-energy houses will also contributeto a reduction in pollution connected with fuel combustion.