The Significance of the Thermal Energy Storage for Efficient

The Significance of Thermal Energy Storage for Efficient Energy Use

Tomasz Kisilewicz1

1Cracow University of Technology, Cracow, Poland

ABSTRACT: The structure of the technically proven building used in simulations was modified to investigate the influence of the space thermal capacity on heating and cooling demand. In every case, the overall heat transmission coefficient of the external walls was kept at the same level to assure comparable conditions of the survey. Results presented in this paper are based on the overall building simulations by means of the Energy Plus program. The air heating and cooling systems allowed to maintain the minimum, continuous internal air temperature at 20oC and maximum not higher than 250C. The high thermal capacity of the space is more important to reduce the overheating rather than to increase the efficiency of the solar energy use. It allows also to minimize the cooling output power of the system. Practically, 10-12cm layers of concrete or brickwork provide enough thermal capacity to reduce the cooling needs. The high thermal stability of the space appears to be, paradoxically, very useful during the colder periods of the summer. It helps to reduce temperature fluctuations and to avoid low internal air temperatures when the heating system is turned off. Conference Topic: 2 Design strategies and tools Keywords: passive solar use, thermal storage

INTRODUCTION One of the most important features of the sustainable building is its significantly reduced consumption of conventional energy and increased use of unconventional energy sources. The passive solar systems still appear to be the most promising and simplest way to absorb, accumulate and utilize solar energy in sustainable buildings. Passive systems have been broadly described and discussed over the past decades. Unfortunately, there is little reliable evidence to support the thermal efficiency and design rules of these systems, especially in the climate of Central Europe. The most common passive system is direct gain, which is in fact applied in every building with glazed openings. The available, in the local climate, solar energy can be used in the most efficient way only when the building design is based from the very beginning on the sustainable solar architecture rules of thumb. The main factors influencing the solar building performance include: - local climate - thermal conductance of the envelope - glazing size and its characteristics - thermal storage capacity. Some of the relations that exist between these factors have been investigated and reported in this paper. The main goal was to formulate the simple rules of thumb for solar designers in Poland.

2. BUILDING SIMULATION Computer simulation of the seasonal thermal behavior of buildings has become an important design tool nowadays. Even the functioning of very complex physical systems like the building itself, heating and ventilating installations, can be simulated step by step under non-stationary environmental and internal conditions. While it is not possible or reasonable to simulate every designed building in this way, this tool may be extremely useful in the case of new technologies, architectural ideas or outstanding energy saving measures. Computer simulation is a valuable research tool, allowing to develop and investigate the precise design rules for engineers. 2.1 Program Energy Plus One of the largest and newest simulation tools is, nowadays, an American program called Energy Plus. The newest version 1.2 was released by the American Department of Energy, its agencies and other cooperating institutions in April 2004. Version 1.1 was used to get the results presented in this paper. The general overview of this program was summarized in [1]. Weather data for Kraków, Poland, in the special Energy Plus EPW format of hourly data, have been imported from the American Society of

Plea2004 - The 21th Conference on Passive and Low Energy Architecture. Eindhoven, The Netherlands,September 2004

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Heating, Refrigeration and Air Conditioning Engineers (ASHRAE) website. 2.2. Main assumptions The simulated building consists of nine thermal zones. All of the zones were considered during the simulation run but only one was the subject of numerous modifications and analyses. It was assumed that: - the entire opaque envelope of the lightweight

building structure is well insulated; thermal conductance of the wall is 0.249W/(m2K), roof 0.165 W/(m2K) and floor 0.245 W/(m2K)

- high density structural layers are in good thermal contact with the space [2,5,6]

- ideal follow-up heating and cooling systems are considered

- the minimum internal air temperature is continuously set to 200C and the maximum to 250C; there is no intermittent heating

- heating simulation period: 15.IX – 15.VI, four time steps in an hour

- ventilation rate is constant and equal to: ca. 0.4 ac/h in the investigated south-west zone

- building is unoccupied i.e. there are no internal heat gains other than sun radiation and the purchased air heating system

- one-dimension heat flow through the building envelope is considered

- the amount of purchased heating and cooling energy is the measure of the passive system’s thermal efficiency.

2. THERMAL STORAGE The thermal capacity of the building space was modified by means of the altered internal massive layers of the walls and the floor. The properties of the internal material layers used in simulations are given in Table I. THERMAL STORAGE PROPERTIES Table I : The properties of the internal accumulating layers used in simulations

Material

Density [kg/m3]

Thickness [m]

Thermal capacity

[kJ/ (m2⋅K)]

Thermalactivity [Ws0.5/ (m2K)]

1 2 3 5 6 concrete 2500 0.2 420 1889 ceramic

brick

1800

0.25

396

1104 concrete 2500 0.10 210 1889 ceramic

brick

1800

0.12

190

1104 plaster 1850 0.02 31 1129

styrofoam 20 0.13 2.2 9.4 In spite of the introduced changes, total heat transmission through the outer walls was in each case

kept at the same level, due to the adjusted insulation thickness. Investigated cases of the heat accumulating layers' arrangement are presented in Table II. In all cases, the light structure of the ceiling is covered with gypsum plaster only. In reference case I, the external walls are covered with 20cm of concrete, the internal walls with 10cm of concrete, the floor with 4cm of concrete and ceramic tiles. THERMAL STORAGE DISTRIBUTION Table II: Analyzed cases of the thermal storage distribution Case Accumulating layers

I* All partitions (except ceiling) with thick internal massive layers

II External walls with concrete 20 cm, the others with 2cm plaster

III External walls with ceramic brick 25 cm, the others with 2cm plaster

IV External walls with concrete 10 cm, the others with 2cm plaster

V External walls with ceramic brick 12 cm, the others with 2cm plaster

VI All partitions with internal plaster only VII** External walls without any massive layer, the

others with 2cm plaster * Reference case, high capacity storage **This case has no practical significance because insulation is in fact always covered with high density layers. It is rather a kind of theoretical extreme case of low capacity storage. 3. SPACE WITHOUT WINDOWS The first run of the simulations was conducted for the space without windows. The aim of this theoretical case was to find out how external heat waves are transmitted through the building envelope and how they influence the heating demand of the space throughout the entire heating season. To increase the significance of the solar heat gain through opaque walls, the absorption coefficient εsol of the external wall surface was set to 0.72. There was practically no influence (less than 0.3%) of the wall thermal capacity on the seasonal space heating demand in cases I to VI. A slightly larger difference (1.8% increase) was observed only in theoretical case VII (lightweight wall). No overheating beyond +250C occurred in the "blind space" during the heating season, however, the maximum value of the MRT (Mean Radiant Temperature) is directly connected with wall thermal inertia: For a wall with a thick internal concrete layer, the seasonal maximum MRT was 20.93oC, 22.72 oC for internal plaster and 26.04 oC in case of no massive layer inside. 4. HEATING ENERGY DEMAND VS. WINDOW AREA


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According to the traditional architectural principle, the adequate window area for natural lighting is 1/10 to 1/8 of the floor area. However, in case of well insulated glazing, a south-oriented window becomes not only a source of light but a source of net heat gain as well [3]. Solar radiation, efficiently absorbed and accumulated in thermal storage allows to reduce space heating demand. The reduction scale depends on many independent factors like: solar radiation intensity, glazing transmission, space thermal capacity, conduction losses etc. Dynamic building simulation allows to investigate relationships between these factors. The south-side windows of the simulated space are glazed with highly insulating low emission glazing filled with xenon. Glazing thermal conductance is 0.912 W/(m2K) and SHGC is 0.331. The calculated heating demand index "E" is a kind of seasonal energy performance label used in this context to evaluate the efficiency of the passive solar system. The value of E shows how much conventional energy per square meter of heated zone must be delivered by the conventional heating system during the entire heating season to maintain internal air temperature at the set level.

HEATING DEMAND

78

80

82

84

86

88

90

92

I II III IV V VI VIIThermal Storage (acc. Table 2)

Hea

ting

inde

x E

[kW

h/(m

2 a)]

1/6 1/3 1/2 >1/2

Figure 1: Seasonal heating demand index vs. south window area and modified internal thermal storage

4.1. Window area 1/6 of the floor area Results of the simulations for south-side windows with areas equal to 1/6 of the floor area are shown in Figure 1, curve "1/6". It must be stressed that the numbers on the vertical axis cover only a small fraction of the entire value, so the graphic differences are in this way dramatically increased. In cases I to V, there is practically no influence of space thermal capacity on heating energy load. It was confirmed that very thick layers (beyond 10-12cm) do not increase the effective space storage abilities. In

case of the thin plaster storage (VI), the efficiency of the passive system is slightly reduced and a further reduction may be observed for case VII. The surprisingly positive results obtained for thin accumulating layers of plaster are in good harmony with Mayo's observations [4]. According to Mayo, the real depth of temperature fluctuations does not exceed 15-30 mm. 4.2. Window area 1/3 of the floor area In the next step, the window area was increased to 1/3 of the space floor area. It resulted in diminished heating load and increased differences between the analyzed cases. Significant solar gains are most efficiently used in the heavy building (case I). A reduction of the thermal capacity results in increased (1.8%) demand for conventional heating energy (case II to V). The irregular inclination of the curve is the result of distinctly diverse thermal conduction and density of concrete and brick, i.e. material thermal activity, table 1, column 6. The low thermal capacity of the plaster layer is connected again with increased heating load (1.6%). A further increase (1.5%) takes place in case VII. It must be noted that the observed differences of the heating index are in fact relatively small compared with significant changes in the space thermal capacity. 4.3. Window area 1/2 of the floor area The increase of the south-side window area to 1/2 of the floor area does not introduce any substantial changes to heating demand. E-value is one percent lower in case of massive walls and one percent higher in case of light external walls compared with a window area equal to 1/3 of the floor area. Solar gains from increased window areas do not result in any further reductions of the seasonal heating load. Oversized window area, as in case ">1/2", results in significantly increased heating demand. The conducted simulations not only allow to investigate the significance of thermal capacity but also help to indicate the optimum window area. It must be observed that the optimal window area is tightly connected with specified thermal mass. A further analysis of this relationship will be conducted and published later this year. At present, it may be assumed that for thermal capacities analyzed in this paper, the optimal window area is within the range of 0.35 to 0.45 of the floor area. A larger window area is related of course to higher space capacity. Thin plaster layers provide reasonable thermal storage, but much thicker heat accumulators improve system efficiency by 6-7% [6]. 5. COOLING LOAD VS. SOUTH WINDOW AREA As described in section 2.2, internal air temperatures could fluctuate between 200C and 250C. The amount of energy that had to be removed from the space by the cooling system was a measure of


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overheating. Diagrams with calculated overheating loads are presented in Fig. 2 The general rising trends of the curves in Fig. 2 look fairly similar to those shown in Fig.1 but this impression could be completely misleading when real values are considered. The maximum overheating value is several times higher than the minimum one. Maximal percentage differences go up even to 600%. There is practically no overheating for a window area equal to 1/6 of the floor area. The curve for this case is located on the horizontal axis. This means that the solar beam and diffused radiation entering the space is immediately used for heating or efficiently absorbed and stored for later use in internal heat accumulators. A non-zero value of overheating load occurs only in case of a very low thermal space capacity (external walls without a massive layer). Solar flux transmitted through a window area 1/3 of the space floor may be efficiently used and accumulated without significant overheating only in a building with a high thermal capacity.

OVERHEATING

020406080

100120140160

I II III IV V VI VIIThermal Storage (acc. to Table 2)

Ove

rhea

ting

[kW

h]

1/6 1/3 1/2 >1/2

Figure 2: Seasonal overheating vs. modified storage capacity and south window area Reduced mass results in overheating load practically the same for cases II to V. It was proved again here, that mass thicker than 10-12cm does not improve the system's thermal efficiency. Further increased window area significantly increases overheating, especially when space heat capacity is radically reduced. 6. HEATING AND COOLING POWER INPUT The thermal capacity of the building space or so called thermal stability influences not only the heating and cooling loads but also the power input of both systems. This feature may often be a very important factor in technical and economic performance, especially in the case of the cooling system.

Maximum calculated heating and cooling power of the ideal systems has been analyzed for a few cases as in Table III, for the extended heating season only. Heating power range is relatively narrow, although ultimate storage cases are considered. The first case responds practically to a theoretical zero thermal capacity. Heat may be stored here only in lightweight insulation layers. Demanded power input in this case is 20% higher than for a high capacity space with thick massive internal layers all around. HEATING AND COOLING POWER Table III: Maximum values of heating and cooling power

Thermal Storage

Maximum heating power

[W]

Maximum

cooling power

[W] No storage inside

(just insulation layer) 1442 2199

Internal plaster only 1396 1488 Massive floor, the walls with 2cm internal plaster

1329 1065

External walls with10cm concrete, the others with

2cm internal plaster

1271 1319

External walls with 20cm concrete, the others with

2cm plaster

1264 1316

Massive internal walls, the others with 2cm plaster

1263 1219

All partitions with thick internal massive layers

1200 817

A building's thermal stability is of critical significance to cooling power input. Low cooling power is demanded for a massive space whereas an 80% increase would be necessary for lightweight technologies with plaster inside and nearly 170% for a zero capacity space. Especially effective thermal stability is achieved with a massive floor due to the direct beam radiation that is absorbed by its surface during spring or autumn days with a high solar altitude. It should be noted again that a 10 cm concrete layer provides enough thermal capacity to minimize the heating and cooling power input; a thicker layer is not practical for improving storage efficiency. 7. SUMMER PERIOD The summer period consists of only four months per year and is not part of the extended heating season. It was assumed that in this period, both the heating and cooling systems are off so the undisturbed free course of inside air temperatures might be observed. Ventilation was kept at the same level as in winter to investigate the influence of the storage on the thermal comfort in the building. Window size was slightly below the optimum: 1/3 of the floor area. The conditions in the analyzed zone are described by:


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- air temperature - mean radiant temperature (MRT) - predicted mean vote (PMV). Three cases of the thermal storage arrangement have been analyzed: A - all the partitions with internal plaster only (as case VI in section 2) B - external walls with a 10cm concrete layer inside, the others with plaster only (as case II) C - all the partitions with thick massive layers inside (as case I). Results of the simulations are presented in Table IV. THERMAL COMFORT Table IV: Thermal comfort evaluation for summer conditions

Thermal Storage

Air temp. range [oC]

MRT range [oC]

PMV range

A 13.70÷32.22 14.56÷31.08 -1.46÷2.15 B 15.08÷30.39 16.31÷28.42 -1.16÷1.66 C 16.23÷28.78 17.64÷26.53 -0.92÷1.30

A positive influence of the thermal storage capacity on internal comfort may be easily observed [7]. A high thermal capacity of the space allows to decrease the range of temperature fluctuations. But in all three cases, the predicted conditions go far beyond the comfort range. A major increase in heat capacity in case C is not connected with a proportional (linear) temperature range reduction. Temperature differences between case B and C are smaller than for cases A and B. Summer thermal comfort is typically associated with overheating but it is easy to observe in Table III that low temperatures may be an object of concern as well. Although internal gains may raise air temperature in a used building by 2 or 3 degrees, it would still be uncomfortably low in a lightweight building (case A). Heat stored in massive building walls allows to maintain thermal comfort during cold periods of the summer without any conventional support. A low capacity building demands heating energy input in the summer.

LOW TEMPERATURES IN SUMMER

5

10

15

20

25

30.VIII - 4.IX

Tem

pera

ture

[oC

]

Ambient air temp..Internal air - massive w allsInternal air - plaster only

Figure 3: Free temperature course during cold summer period Temperature diagrams for the coldest summer period have been compiled in Fig. 3. The curves are thick because over 140 data points have been set together in a small picture. Over a few consecutive cold and cloudy days, internal heat accumulators become gradually discharged, thus allowing the air temperature to drop to a minimum (see arrow). Air temperature in the light building is constantly lower than in the other building. The mean inclination of the temperature curves depends on space thermal capacity and is distinctly different for both cases. The reverse process of charging accumulators starts again on the first sunny day. Air temperature in the light space rises quickly above the air temperature in the massive building, thus increasing overheating intensity. Comprehensive comfort evaluation is connected not only with temperature range but also with overheating frequency distribution. In case of external walls with massive concrete layers inside, temperatures higher than 250C will last for 283 hours, and higher than the limit value of 270C for 80 hours, Fig.4. Such overheating would be extremely troublesome for users. In practice, intensive ventilation and air exchange with the other building zones would significantly reduce overheating range and duration.

AIR TEMPERATURE FREQUENCY


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0

100

200

300

400

500

13 15 17 19 21 23 25 27 29 31 33 35Internal air temperature [oC]

Freq

uenc

y di

strib

utio

n

Figure 4: Frequency distribution of internal air temperature, (external walls with 10cm concrete) Summer overheating risk is closely related to storage capacity. Higher storage capacity matches the lower values of maximum air or radiant temperatures and the higher temperatures during cold periods. The best results have been obtained for the space with thick and massive partitions. CONCLUSIONS Results presented and discussed in the paper allow to formulate below a few simple rules of thumb for solar architecture designers. The importance of these rules is connected with the investigated relationships between building characteristics and the local climate. These rules are valid for buildings located in the southern regions of Poland, wall and roof U value below 0.26 W/m2K and insulating glazing with LE coating: The broader range of data usability demands further investigation. 1. For the above conditions, the optimum south

window area, in order to minimize conventional energy use for space heating, should be close to 1/3 of the floor area.

2. A large area of the thick massive walls - as could be expected - ensures the most efficient storage to accumulate momentary surplus of solar energy for later use. On the other hand, highly reduced space thermal capacity increases energy demand merely by 4-7%.

3. Cooling load is extremely sensitive to space thermal capacity. A large window area demands high thermal capacity to minimize cooling and space overheating.

4. Sufficient thermal storage to minimize heating and cooling loads is provided by massive wall layers 10-12 cm thick; thicker layers have practically no influence on storage efficiency.

5. Further efficiency increase may by achieved only by means of the increased storage area.

6. The high thermal stability of the building allows to minimize the heating and especially cooling power input .

7. A vast majority of Polish building technologies are based on thick, high density walls and floors,

providing enough thermal storage to use efficiently solar energy and to minimize overheating. Light building technologies, with wooden or light metal framework, may need extra thermal storage.

REFERENCES [1] T. Kisilewicz, Indirect Passive Solar Use in Central Europe – Preliminary Studies, Sustainable Building 2002 International Conference, Oslo 2002. [2] E. Kossecka, J. Kosny, Influence of insulation configuration on heating and cooling loads in a continuously used building, Energy & Buildings 34 (2002) 321-331. [3] T. Kisilewicz, Building thermal response to the solar energy load, International Civil Engineering Conference: Budova a Energia IV, Kosice, Slovakia 2001. [4] T. Mayo Residential Passive Solar Design: A Guide to the Technology, National Research Council Canada, Ottawa 1986. [5] E. Kossecka, J. Kosny: The Effect of Structure of Exterior Walls on the Dynamic Thermal Performance of a whole Building, IV Polish Scientific and Technical Conference ENERGODOM 2000, Kraków-Mogilany 1998. [6] T. Kisilewicz: Passive solar savings in Central Europe climate - Abstract, IV-th International scientific practical conference: Problems of energy saving, Lwów 2003 [7] J. Kosny, P. Childs, T. Petrie, A. Desjarlais, D. Gawin, J. Christian: Energy Benefits of Application of Massive Walls in Residential Buildings, Conference proceedings: Performance of Exterior Envelopes of Whole Buildings VIII, Florida - USA 2001


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The Significance of the Thermal Energy Storage for Efficient