Heating and cooling systems for better energy …Heating and cooling systems for better energy efficiency Bjarne W. Olesen, Ph.D. International Centre for Indoor Environment and Energy,

Heating and cooling systems for better energy efficiency Bjarne W. Olesen, Ph.D.

International Centre for Indoor Environment and Energy, Department of Mechanical Engineering, Technical University of Denmark. Corresponding email: [email protected] SUMMARY Heating, ventilation and cooling of buildings is responsible for 30-40 % of the energy consumption in buildings and a corresponding significant amount of CO2 emission. Since 2006 the European Energy Performance of Buildings Directive (EPBD) is being implemented in building codes on a national level. For new and existing buildings this requires a calculation of the energy performance of the building including heating, ventilation, cooling and lighting systems, based on primary energy. Each building must have an energy certificate and regular inspections of heating, cooling and ventilation systems must be performed. This has increased the focus on energy efficiency of systems and there components. The concept of the European standards developed to support the EPBD is presented in this paper. These standards may include alternative calculation methods and many European countries are using national methods. This means the heating system may be evaluated differently in different countries. In Northern Europe a heating system is still needed even if the insulation of buildings will increase significantly. However, in middle Europe the increased insulation may result in a very low heat demand and a full heating system may not be needed. Also global warming will in the future impact the need for a heating system in many geographical areas. This may lead to alternative solutions for heating of buildings. On the other hand peoples wish for more comfort, future higher outside temperatures, heat islands in big cities will increase the use of cooling systems. Further more we must increase the use of renewable energy sources. This requires systems, which can be used for heating at relative low temperature (air-water) and used for cooling at relative high system temperatures. The paper will present studies on the use of water based radiant heating and cooling systems with water temperatures close to room temperatures. The paper also introduces the concept of exergy for evaluation of sustainable heating and cooling systems. INTRODUCTION In 2003 the European Commission (EC) issued a directive, 2002/91/EC [1]. The objective of this directive is to promote the improvement of the energy performance of buildings within the community, taking into account outdoor climatic and local conditions, as well as indoor climate requirements and cost-effectiveness. The directive refers to the energy use and dos not take into account the life cycle energy demand (energy used to produce the products used for the building) For new and existing buildings this requires a calculation of the energy performance of the building including heating, ventilation, cooling and lighting systems, based on primary energy. Each building must have an energy certificate and regular inspections of heating, cooling and ventilation systems must be performed. This directive required all member countries by January

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2006 to implement the directive in the building codes on a national level. Until now this has only been implemented fully in a couple of countries.

CONCEPT OF THE EPBD-STANDARDS A mandate to the European Organization for standardization (CEN) from the Commission, M343-EN-2004 [2] was issued. This mandate asks CEN to elaborate and adopt standards for a methodology, calculating the integrated energy performance of buildings and estimating the environmental impact, in accordance with the directive. To coordinate the standardization related to the EPBD, CEN established an EPBD-Project Group including the following Technical Committees (TC's):

TC 89 Thermal performance of buildings and building components; TC156 Ventilation for buildings; TC169 Light and Lighting; TC228 Heating systems in buildings; TC247 Building automation, controls and building management

The standards under the mandate shall constitute an integrated and interacting methodology for the calculation of the energy uses and losses for heating, cooling, ventilation, domestic hot water and lighting systems, taking into account natural lighting, passive solar systems, passive cooling, position and orientation, automation and controls, and auxiliary installations necessary for maintaining a comfortable indoor environment. The methodology shall integrate, where relevant, the positive influences of active solar systems and heat and electricity from renewable energy sources, as well as quality co-generation heating plants (CHP, including micro-CHP) and district heating and cooling systems. It should also facilitate an estimation of the environmental impact from this energy use and provide data requirements for carrying out standard economic evaluations for the use of different systems. This series of standards (about 40) have or are being voted for as final standards. The objective is to establish common calculation methods in Europe for energy performance of buildings and HVAC systems. Unfortunately this did not happened because the standardization work started too late and several countries have adopted national calculation methods. Also some of the standards do include alternative methods, which mean the energy performance of the same system may be evaluated differently in different countries. Energy performance of heating systems A basic standard for the calculation of the building energy demand (EN ISO 13790 [3]) will form the central point of the calculation procedure. To perform this calculation, input data for indoor climate requirements, internal loads, building properties and climatic conditions are needed. Standards and methods for these input data exist already to a great extent. The calculation of the building energy demand does not take into account the heating-cooling-ventilation system. The calculated building energy demand serves then as an input to the calculation of the system energy requirement. The boundary between building and system is shown in Figure 1 for a heating system. The additional losses are calculated for heat emission, distribution, storage and generation. The auxiliary electrical energy needed for fans, pumps etc. will also be calculated. The effect of the control system is included in the building energy demand as well as the additional losses from the system due to sub-optimal control. The additional energy savings obtained with a whole

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building automation system (heating, cooling, ventilation, electrical appliances, light etc.) will be taken into account in a separate standard (EN 15232[4]). Output from the calculation (Figure 1) will be the net energy (building energy demand) together with the required heating/cooling ventilation energy for the HVAC systems, including the auxiliary energy. Finally, the total delivered energy for the building/system can be calculated by adding the required energy for all the systems, including lighting. This will be converted to Primary Energy, taking into account renewable energy sources and national conversion factors. The calculation process comprises three basic points, which are, calculation of net energy (building energy demand), calculation of delivered energy (system energy demand), and conversion to primary energy. Delivered energy takes into account the losses coming from the heat emission, heat distribution and heat generation system.

Generation

Emission Distribution Storage

QgQsQ d

Q c,e

Final energy (boundary:building)

Primary energy

Calculation direction (from the demand to the source)

heat demand Q V

Q h

Q T

QS

Q i Davi

Energy direction (from the source to the demand)

Figure 1. Calculation concept and building-system boundaries for heating (EN15316-1 [5])

Heat losses for the heat emission system (EN15316-2.1) Emission losses are due to three factors, namely, non-uniform temperature distribution, losses to the outside from embedded heating devices in the structure, and losses due to non-perfect control of the indoor temperature. The heat energy losses of heat emission are calculated as: Qem,ls = Q em,str + Q em,emb + Q em,ctr [J] (1) where: Q em,str heat loss due to non-uniform temperature distribution in Joule (J); Q em,emb heat loss due to emitter position (e.g. embedded) in Joule (J); Q em,ctr heat loss due to control of indoor temperature in Joule (J). Two methods are recommended in the standard. The two methods do not give exactly the same results, but the same trend. The two methods shall not be mixed.


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Method using efficiencies of the emission system The evaluation of Qem,ls takes place monthly or by another time period in accordance with equation (2).

Hlsem,

hydrintRadiantlsem, 1 Q

fffQ ⎟

⎟⎠

⎞⎜⎜⎝

⎛−=

η (2)

where Qem,ls is the additional loss of the heat emission (time period), in kWh; QH is the net heating energy (time period) (EN ISO 13790), in kWh; fhydr is the factor for the hydraulic equilibrium. fim is the factor for intermittent operation (as intermittent operation is to be understood the time-dependent option for temperature reduction for each individual room space); frad is the factor for the radiation effect (only relevant for radiant heating systems); η em is the total efficiency level for the heat emission in the room space. The total efficiency level η em is fundamentally evaluated as

))(4(1

eembctrstr

m ηηηη

++−= (3)

where ηstr is the part efficiency level for a vertical air temperature profile; ηctr is the part efficiency level for room temperature control regulation; ηemb is the part efficiency level for specific losses of the external components (embedded systems). In individual application cases this breakdown is not required. The annual expenditure for the heat emission in the room space is calculated as

∑= lsem,als,em, QQ (4) where Qem,ls,a is the annual loss of the heat emission, in kWh; Qem,ls is the loss of the heat emission (in the time period) in accordance with equation (2), in kWh. Default values fro the different efficiencies and factors can be found in an informative annex to the standard. Some of these values are based on real data from experiments and/or computer simulations, while others are made by agreement. Examples of the values included in the annexes are given in table 1 to 3.

Method using equivalent increase in internal temperature The internal temperature is increased by:

• The spatial variation due to the stratification, depending on the emitter; • The control variation depending on the capacity of the control device to assure a

homogeneous and constant temperature. The equivalent internal temperature, θint,inc taking into account the emitter, is calculated by:

ctrstriniinc θθθθ Δ+Δ+= int,int, (°C) (5) where: θint,ini initial internal temperature (°C);


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Δθstr spatial variation of temperature; Δ θctr control variation. The influence of an equivalent increase in internal temperature. of the heat emission system may be calculated in two different ways: • by multiplying the calculated building heat demand, QH, with a factor based on the ratio

between the equivalent increase in internal temperature, Δθint,inc, and the average temperature difference for the heating season between the indoor and outdoor temperature for the space: Qem,ls = QH · ( 1 + Δθint,inc / (θint,inc - θe,avg) ) [J] (6)

• by recalculation of the building heat energy requirements, according to EN ISO 13790, using the equivalent increased internal temperature. as the set point temperature of the conditioned zone. This second approach leads to a better accuracy.

For ηstr an average value is to be formed from the data for the main influence parameters "over-temperature" and "specific heat losses via external components".

ηstr = (ηstr1 +ηstr2)/2

Table 1. Efficiencies for free heating surfaces (radiators); room heights ≤4 m.

Efficiencies Influence parameters

ηstr ηctr ηemb

Room space temperature regulation

unregulated, with central supply temperature regulation Master room space P-controller (2 K) P-controller (1 K) PI-controller PI-controller (with optimisation function, e.g. presence management, adaptive controller)

ηstr1 ηstr2 Over-temperature (reference Θi = 20 °C)

60 K (e.g. 90/70) 42.5 K (e.g. 70/55) 30 K (e.g. 55/45)

0.88 0.93 0.95

0.800.880.930.950.970.99

specific heat losses via external components (GF = glass surface area)

radiator location internal wall radiator location external wall - GF without radiation protection - GF with radiation protectiona - normal external wall

0.87

0.83 0.88 0.95

1

1 1 1

a The radiation protection must prevent 80% of the radiation losses from the heating body to the glass surface area by means of insulation and/or reflection.

EXAMPLE: radiator external wall; over-temperature 42.5 K; P-controller (2 K) ηstr = (ηstr1 +ηstr2)/2 = (0.93 + 0.95)/2 = 0.94; ηctr = 0.93; ηemb = 1 ηem = 1/(4 – (0.94 + 0.93 + 1)) = 0.88 Factor for intermittent operation fim = 0.97 Factor for radiation effect: frad = 1.0


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Table 2. Factor for hydraulic balancing: fhydr.

Influencing parameters Factor for

hydraulic balancing, fhydr

non balanced systems 1.03 Hydraulic balance

Signed balancing report and in compliance with EN 14336 • more than 8 emitters per automatic differential

pressure control or only static balanced systems

1.02

Signed balancing report and in compliance with EN 14336,

• Max 8 emitters per automatic differential pressure control

1.00

For ηemb an average value is to be formed from the data for the main influence parameters "system" and "specific heat losses via laying surfaces".

ηemb = (ηemb1+ ηemb2)/2 (A4)

Table 3. Efficiencies for component integrated heating surfaces (panel heaters); room heights ≤4m.

Part efficiencies influence parameters

ηstr ηctr ηemb

Room space temperature regulation

Heat carrier medium water - unregulated - unregulated, with central supply temperature regulation - unregulated with average value formation (ϑV –ϑR) - Master room space - two-step controller/P-controller - PI-controller Electrical heating -two-step controller - PI-controller

0.75 0.78 0.83 0.88 0.93 0.95

0.91 0.93

ηemb1 ηemb2System Floor heating - wet system - dry system - dry system with low cover Wall heating Ceiling heating

1 1 1

0.96 0.93

0.930.960.980.930.93

Specific heat losses via laying surfaces

Panel heating without minimum insulation in accordance with DIN EN 1264 Panel heating with minimum insulation in accordance with DIN EN 1264 Panel heating with 100% better insulation than required by DIN EN 1264

0.860.95

0.99


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EXAMPLE: Floor heating - wet system (water); two-step controller; floor heating with high level of heat protection ηstr = 1.0; ηctr = 0.93; ηemb = (ηemb1+ ηemb2)/2= (0.93 + 0.95)/2 = 0.94 ηem = 1/(4 – (1.0 + 0.93 + 0.94)) = 0.88 Factor for intermittent operation: fim = 0.98 Factor for radiation effect: frad = 1.0 Factor for hydraulic balancing: fhydr same as for radiators Heat losses for the heat distribution and generation system The heat losses of a distribution system depend on the average temperature of the heating medium, the temperature of the surrounding envelope, length and insulation of the pipes. For the heat losses in a time step the following formula applies: [J] (10) H

iamD tLUQ ⋅⋅−⋅= ∑ )(' ϑϑ

where: 'U U-value per length, [W/mK]

mϑ average medium temperature, [°C]

aϑ surrounding temperature, [°C] L length of the pipe [m]

Ht heating hours in the time step, [h] The standard EN15316-2.3 gives both approximated and detailed methods. The most important factors are the temperature level of the heating medium. Low temperature heating will result in decreased losses. The calculation of losses from heat generation systems like boilers, heat pumps, CHP, district heating, solar heating and biomass combustion systems are considered in other standards. Also here a lower water temperature will result in increased energy efficiency. Figure 2 show an example of the energy performance calculation for a one-family house for a radiator and a floor heating system. It can be seen that the additional losses from the heating system contributes with about 20% to the total primary energy.

Figure 2. Losses in primary energy for a radiator and a floor heating system.


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EMBEDDED WATER BASED SYSTEMS FOR LOW TEMPERATURE HEATING AND HIGH TEMPERATURE COOLING In Europe it is mainly water-based heating systems that are used. These systems use radiators or floor heating as heat emitters. One advantage compared with air systems is the more efficient means of transporting energy. The demand for comfort, better insulation of buildings, and greater internal loads from people and equipment have increased interest in installing also a cooling system to keep indoor temperatures within the comfort range. This resulted first of all in the introduction of suspended ceiling panels for cooling and in recent years also in the use of floor systems for cooling (Simmonds et al. [8]; Olesen [9]). Typical positioning of pipes for wall, floor and ceiling systems is shown in Figure 3. A new trend, which started in the early nineties in Switzerland (Meierhans [10]), is to use the thermal storage capacity of the concrete slabs between each storey in multi-storey buildings. Pipes carrying water for heating and cooling are embedded in the centre of the concrete slab (Figure 3). By activating the building mass, you will not only get a direct heating-cooling effect, but you will also, due to the thermal mass, reduce the peak load and transfer some of the load outside the period of occupancy. Because these systems for cooling operate at a water temperature close to room temperature, they increase the efficiency of heat pumps, ground heat exchangers and other systems using renewable energy sources.

Figure 3. Examples of the positioning of pipes in floor, wall, ceiling and slab. Even if surface heating and cooling systems often have a higher thermal mass than other heating/cooling systems, they have a high control performance. This is partly due to the small temperature difference between the room and the system (water, surface) and the resulting high degree of self-control. Studies on controllability of floor heating/cooling (Olesen [11]) show that floor heating control the room temperature as good as radiators. To avoid condensation on a


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cooled surface, there is a need to include a limitation on water temperature, based on the space dew-point temperature. Design and dimensioning of these systems including calculation of heating and cooling capacity can be done according to the new standards EN15377-1 and 2. [12, 13] Thermo Active Building Systems (TABS) A Thermo-Active-Building-System (TABS) is a water based heating and cooling system, where the pipes are embedded in the central concrete core of a building construction. The heat transfer takes place between the water (pipes) and the concrete, between the concrete core and the surfaces to the room (ceiling, floor) and between the surfaces and the room. The peak-shaving is the possibility to heat and cool the structures of the building during a period in which the occupants may be absent (during night time), reducing also the peak in the required power (Figure 4). In this way energy consumption may be reduced and lower night time electricity rate can be used. At the same time a reduction of the size of cooling system including chillier is possible. The performance and dimensioning of TABS can be done by full dynamic building simulations with commercial programs including calculation models for embedded pipes. (Olesen and Dossi, [14]). A study was performed with the aid of the dynamic simulation program. The system considered is shown in Figures 5 and 6. The meteorological ambient boundary conditions correspond to those of Würzburg/Germany and Venice/Italy. The external temperature data for winter and summer design days are shown in Table 3. Summer was the period from 1 May to 30 September, and winter was the period from 1 October to 30 April

Figure 4. Example of peak-shaving effect (X-axes: time; y-axes: cooling power W ) 1) heat gain, 2) power needed for conditioning the ventilation air, 3) power needed on the water side, 4) peak of the required power reduction


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Roomwidth = 3.6 m

Detail

Office Corridor Office

Figure 5. Central room module used for the computer simulation of a building with concrete slab cooling. All dimensions are in metres. The time of occupancy was Monday to Friday from 8.00 to 17.00, with a lunch break from 12.00 to 13.00. The system was in operation only outside the period of occupancy, from 18:00 to 06:00. Internal heat sources: during occupied periods corresponding to 27.8 W/m2. Ventilation (ach): outside time of occupation 0.3 h-1 (infiltration); during occupation 1.5 h-1 (~ 11 l/s per person). Sun protection: during occupation, by direct exposure of sunlight and operative temperature above 23°C, reduction factor z = 0.5.

igure 6. Position of the plastic pipes in the concrete slab between two storeys.

able 4: Design day outdoor temperatures for Würzburg, Germany and Venice, Italy.

Heating Dry Bulb Cooling Dry Bulb

F T

[°C] [°C] City Lat. Long. Elev.

99% 2% [°] [°] [m] 99,6% 0,4% Venice 0 N 0 E 45.3 12.2 6 -4,9 -3,1 30,8 28,2Würzburg-Frankfurt 50.05 N 8.60 E 113 -11 -8,2 30,3 26,7


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The goal for the system used in the study was to operate water temperatures as close to the room

the present study, the supply water temperature was controlled so that it was not lower than

sible

stead of controlling the supply water temperature it may be better to control the average water

or ill

well designed buildings with low heating and cooling loads it may be possible to operate the

upply water temperature is a function of outside temperature according to the equation:

temperature as possible. If very high or very low water temperatures are introduced into the system it may result in over-heating or under-cooling. Inthe dew point in the space. For this purpose, a humidity balance (latent loads from people, outside humidity gain from ventilation) was also included in the simulation. It was then posto calculate the dew point in the room for each time step in the simulation. Intemperature. The return water temperatures are influenced by the room conditions. By maintaining a constant supply water temperature, an increase in internal loads from suninternal heat sources will increase the return temperature. The average water temperature wthen increase and the cooling potential will decrease. If, instead, the average water temperature(½(treturn – tsupply)) is controlled, an increase in return temperature will automatically be compensated for by a decrease in supply water temperature. Insystem at a constant water temperature. The following concepts for water temperature control were studied: S ( ) )22(*6,12020*52,0sup −−+−= operativeply ttt °C (case 801) external

Average water cordin to: temperature is a function of outside temperature ac g ( ) )22(*6,12020*52,0 −−+−= operativeaverage ttt °C (casexternal e 901)

temperature equal to: 22°C in summer and 25°C in winter. (case 1201) :

Average water Supply water temperature is a function of outside temperature according to the equation

( ) 1818*35,0sup +−=ply tt °C summer (case 1401) external

( ) 1818*45,0sup +−= externalply tt °C winter (case 1401)

he results of the simulation are shown in Table 5 for summer conditions and in Table 6 for r

ve

ot s

rgy

Twinter conditions. The operative temperature of the cases 0801, 0901 and 1401 (Table 5) is fomost of the time (>85%) in a comfort range (22-26°C). In Würzburg, 27°C is never exceeded and 26°C is exceeded less than 5% of the time. In Venice, only 5% of the temperatures are abo27°C. The difference between controlling the supply water temperature (case 0801) or the average water temperature (case 0901) is very small. In the case of 1401, the control does ntake into account the internal operative temperature, but the results are almost identical to case0801 and 0901. With a constant average water temperature (22°C), the cooling effect is too low and the operative temperature is often too high (60% of the time above 27°C in Venice and 27% in Würzburg). The energy use is the same for the cases 0801, 0901 and 1401 in Venice. For Würzburg, case 1401 is the energy use, but it is about 10% lower than case 801 and 901. Eneuse in case 1201 with a constant water temperature is relatively high. The pump running time for case 1401 is equal to or lower than for the other cases. In the summer, case 1401 is overall better than the others. Due to the warmer climate in Venice (Table 5) the room temperatures are higher, and energy use and pump running time are also higher compared to Würzburg.


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Table 5: Operative temperatures, temperature drift, pump running time and energy transfer for different water temperature control strategies. Summer conditions. Dead-band 22–23°C. Ventilation rate: 0.3 ach from 17:00 to 8:00, 1.5 ach from 8:00 to 17:00. May to September Time of operation 18:00-06:00 Venice Würzburg

Water temperature control

Supply =F (outside) 0801

Average = F(outside) 0901

Average = 22°C1201

Average=F (outside) 1401

Supply=F (outside) 0801


Average= 22°C1201


°C % % % % % % % % <20 0 0 0 0 0 0 0 0 20-22 0 0 0 0 3 3 1 5 22-25 56 58 8 56 75 78 30 77 25-26 26 25 13 25 18 16 21 14 26-27 13 12 19 14 5 4 22 4

Operative temperature interval

>27 5 5 60 5 0 0 27 0 Pump running hours 1254 1190 1417 1214 1091 971 1327 953

% of time 34 32 39 33 30 26 36 26

Energy Cooling 1104 1109 1297 1106 763 785 978 749 KWh Heating 1 2 0 0 29 41 2 2 Standard EN15377-3 [15] includes more simplified methods to evaluate the dynamic performance of TABS. Besides the standard includes diagrams like the one shown in Figure 7 [16]. This simplified diagram give the relation between internal heat gains, water supply temperature, heat transfer on the room side, hours of operation and heat transfer on the water side. The diagrams correspond to a concrete slab with raised floor (R=0.45 m2K/W ) and a permissible room temperature range of 21 °C to 26 °C. The upper diagram shows on the y-axis the maximum permissible total heat gain in space (internal gains plus solar gains) W/m2, and on the x-axis the required water supply temperature. The lines in the diagram correspond to different hours of operation (8h, 12h, 16h, and 24h) and different maximum amount of energy supplied per day Wh/m2 d. The lower diagram shows the cooling power W/m2 required on the water side (for dimensioning of chiller) for thermally activated slabs as a function of supply water temperature and operation time. Further, the amount of energy rejected per day is indicated Wh/(m2 d). The example shows, that by a maximum internal heat gain of 38 W/m2 and 8 hour operation, a supply water temperature of 18,2 °C is required. If, instead, the system is in operation for 12 hours, a supply water temperature of 19,3 °C is required. In total, the amount of energy rejected from the room is app. 335 Wh/m2 per day. The required cooling power on the water side is by 8 hours operation 37 W/m2 and by 12 hours operation only 25 W/m2. Thus, by 12 hours operation, the chiller can be much smaller. The total heat rejection on the water side is app. 300 Wh/m2 per day.


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Table 6: Operative temperatures, temperature drift, pump running time and energy transfer for different water temperature control strategies. Winter conditions. Dead-band 22–23°C. Ventilation rate: 0.3 ach from 17:00 to 8:00, 1.5 ach from 8:00 to 17:00. October to April Time of operation 18:00-06:00 Venice Würzburg

Water temperature control

Supply=F (outside)0801


Average= 25°C 1201

Average= (outside) 1401

Supply= F (outside) 0801


Average= 25°C 1201


°C % % % % % % % % <20 0 0 0 1 0 0 4 4 20-21 1 1 6 14 9 7 19 24 21-23 72 75 50 63 77 80 50 63 23-24 14 15 5 14 8 7 7 7 24-26 12 10 23 8 6 5 15 2

Operative temperature interval

>26 0 0 16 0 0 0 5 0 Hours 837 642 1487 1166 813 664 1533 1322 Pump

running % of time 16 13 29 23 16 13 30 26

Energy Cooling 144 144 143 143 57 64 63 45 KWh Heating 551 554 407 421 816 834 684 717 EXERGY ANALYSIS "Energy saving" and emission reduction are both affected by the energy efficiency of the built environment and the quality of the energy carrier in relation to the required quality of the energy. Taking into account qualitative aspects of energy leads to introduction of the exergy concept in comparison of systems, which is the key idea of IEA-Annex 37. Energy, which is entirely convertible into other types of energy, is exergy (high valued energy such as electricity and mechanical workload). Energy, which has a very limited convertibility potential, such as heat close to room air temperature, is low valued energy. Low exergy heating and cooling systems use low valued energy, which is delivered by sustainable energy sources (e.g. by using heat pumps, solar collectors, either separate or linked to waste heat, energy storage etc.). Common energy carriers like fossil fuels deliver high valued energy. The reason for "energy saving" being in quotation marks in the first sentence, is that we actually are talking about saving exergy, not energy! Future buildings should be planned to use or to be suited to use sustainable energy sources for heating and cooling. One characteristic of these energy sources is that only a relatively moderate temperature level can be reached, if reasonably efficient systems are desired. The development of low temperature heating and high temperature cooling systems is a necessary prerequisite for the usage of alternative energy sources. The basis for the needed energy supply is to provide occupants with a comfortable, clean and healthy environment.


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Figure 6 – Working principle of TABS (Koschenz and Lehmann [16]. Figure 7 is showing an example of a calculated energy flow and exergy flow from primary energy, heat generator, emission in room and loss to the outside. The figures show that a low temperature heating system will result in less exergy consumption.


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Figure 7. Examples of exergy and energy flow for three type of heating systems. (IEA-Annex 37).

0

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Boiler+radiator

DISCUSSION AND CONCLUSIONS New demands for lowering the energy consumption of buildings require increased insulation of houses, tight constructions with mechanical ventilation and heat recovery. A trend for residential building is the concept of passive houses, where a “normal” heating system may not be needed. This trend will result in much smaller heating systems and in many cases systems using air as the energy transport medium. Also the global warming will result in less need for heating. Because of the EPBD all European countries will in a couple of years have implemented a requirement for calculation or estimation (measurements/calculation) of the energy performance of new and existing buildings. With the development of the new CEN standards related to the EPBD it will in the future be possible to use common method and avoid that the same building system will be evaluated differently. While it is relative well established how to calculate the energy performance of the building alone, there is a lack of data on energy performance of HVAC systems. Especially for cooling systems the calculation methods are not well established. Many manufacturers will try to show that their system has a high energy performance and will also aim at developing more efficient systems and controls. But if the energy performance evaluation methods used are not detailed enough, new development may not be reflected in the calculations. On the other hand detailed calculations may make the methods more complicated. Another trend in Europe is the increasing need for cooling. Peoples increased comfort requirements (air conditioning in cars), increasing thermal loads in many office buildings, global warming, heat islands and the aim for optimal productivity of people, are all issues that will result in a higher demand for cooling. In middle Europe a trend to use water based cooling systems at high water temperature is a way of making the cooling more sustainable with less energy consumption compared to full Air Conditioning. Some of theses systems also use the building mass (TABS) to reduce the peak cooling load and transfer some of the cooling from daytime to night. This trend has resulted in new CEN standards for design and dimensioning of systems and evaluation of the dynamic effect. Also new research studies are looking at the performance and optimal control of such systems. Procedures for calculating the steady-state heating/cooling capacity are available. By a proper control the risk for condensation on the cooled surfaces can be limited. The results of a dynamic computer simulation of different control concepts for a water-based radiant cooling and heating system with pipes embedded in the concrete slabs have been presented. The system was studied


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for both the summer period May to September and the winter period October to April in two geographical locations, Venice, Italy and Würzburg, Germany. The best performance regarding comfort and energy is obtained by controlling the water temperature (supply or average) as a function of outdoor temperature. There is no need to take into account the room temperature. The system was able to keep the room temperatures within a comfortable range, in both summer (cooling) and winter (heating), and in both climatic zones. Due to the use of water temperatures close to room temperatures water based surface heating and cooling systems will increase the possibility to use renewable energy sources like ground source heat pumps, ground heat exchangers, geothermal energy, solar energy, evaporative cooling etc. The level of water temperatures used also increase the efficiency of boilers, chillers and heat pumps. REFERENCES

1. EU-Directive 2002/91/EC of the European Parliament and of the council of 16 December 2002 on the

energy performance of buildings. European Commission 2. CEN M 343 - EN –2004. Mandate to CEN, CENELEC and ETSI for the elaboration and adoption of

standards for a methodology calculating the integrated energy performance of buildings and estimating the environmental impact, in accordance with the terms set forth in Directive 2002/91/EC

3. EN13790-2007. Thermal performance of buildings - Calculation of energy use for space heating and cooling

4. EN15232-2007. Calculation methods for energy efficiency improvements by the application of integrated building automation system

5. EN15316-1-2007. Heating systems in buildings - Method for calculation of system energy requirements and system efficiencies - Part 1: General

6. EN15316-2.1-2007. Heating systems in buildings- Method for calculation of system energy requirements and system efficiencies - Part 2-1: Space heating emission systems

7. EN15316-2.3-2007. Heating systems in buildings - Method for calculation of system energy requirements and system efficiencies - Part 2-3: Space heating distribution systems

8. Simmonds, P., Gaw, W., Holst, S., Reuss, S. (2000) Using radiant cooled floors to condition large spaces and maintain comfort conditions, ASHRAE Transactions, Part I

9. Olesen, B.W. (1997, Possibilities and limitations of radiant floor cooling,. ASHRAE Transactions. V.103, Pt.1

10. Meierhans R.A. (1996) Room air conditioning by means of overnight cooling of the concrete ceiling. ASHRAE Transactions,. V. 102, Pt. 2

11. Olesen, B.W.(2001) Control of floor heating and cooling systems“ . Clima 2000/Napoli 2001 World Congress – Napoli, September 2001

12. EN15377-1, 2007: Design of embedded water based surface heating and cooling systems: Determination of the design heating and cooling capacity

13. EN15377-2, 2007: Design of embedded water based surface heating and cooling systems: Design, Dimensioning and Installation

14. Olesen, B.W. and Dossi, F.C. Operation and control of activated slab heating and cooling Systems, CIB World Building Congress 2004

15. EN15377-3, 2007: Design of embedded water based surface heating and cooling systems: - Part 3: Optimizing for use of renewable energy sources

16. Koschenz, M und Lehmann,B : Thermoaktive Bauteilsysteme, tabs . EMPA, Switzerland, 2000 .


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Documents

Heating and cooling systems for better energy …Heating and cooling systems for better energy efficiency Bjarne W. Olesen, Ph.D. International Centre for Indoor Environment and Energy,