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A Finnish low energy house
Heat Recovery Systems Vol. 5, No. 4, pp. 373-382, 1985 Printed in Great Britain.
0198-7593/85$3.00+ .00 Pergamon Press Lid
A FINNISH LOW ENERGY HOUSE TIMO KALEMA Technical Research Centre of Finland, Laboratory of Heating and Ventilating, 02150 Espoo, Finland JAAKKO KARVINEN Makrotalo Ltd, Kornetintie 4, 00380 Heisinki, Finland and RaSTO CASTR~q Retermia Ltd, Kontiokatu 2 D, 15950 Lahti, Finland
AIm~'t--In the Finnish climate the annual heating energy demand in a modern small house, where the floor area is about 120m2, is 60-70 GJ. The energy includes both the space and the tap water heating. With a new heating system in an extremely well insulated house the annual energy consuAnption is 27 GJ. The heating system includes heat recovery from exhaust air with a heat pump and from sewage water with a thermosyphon. The heating costs are very low. When the mean price of electricity is 11 USD/GJ, the annual heating costs (without capital costs) are about 300 USD.
NOMENCLATURE c E h Q T I~ U e
specificheat [J ks- l K-'] electricalenergy [J] enthalpy [J kg- l] heat energy [J] temperature [K, °C] air volume flow [m3S-I] over-all heat transfer coefficient [W K -~ m -2] coeffident of performance efficiency p air density [kg m-3] heat effect [W] A difference COP coefficient of performance Subscripts
a c cp cth
e ea ia th
air heat pump condenser compressor thermosyphon condenser heat pump evaporator exhaust air indoor air thermosyphon INTRODUCTION
The energy c o n s u m p t i o n o f buildings m a y be decreased with a g o o d insulation o f building envelope and with heat recovery f r o m exhaust air and f r o m sewage water. T h e Finnish building c o m p a n y M a k r o t a l o Ltd has developed with Technical Research Centre o f Finland and Retermia Ltd a low energy house, where these principles are used. The p u r p o s e was to develop a small house, where the energy c o n s u m p t i o n is low without bargaining o f the living quality (ventilation, w a r m water c o n s u m p t i o n and i n d o o r air temperature). The house should be such, that it could be placed everywhere. This d e m a n d m e a n t that the earth heat p u m p could not be used, because the earth pipes take quite a big area. Also solar energy could not be used, because the Finnish climate is such, that small solar heating systems w o r k very p o o r l y [1, 2]. The best energy saving m e t h o d s seemed to be the g o o d insulation o f building envelope and the heat recovery f r o m exhaust air and sewage water. 373
374
TXMO KAt.EMA et al.
j
Fig. 1. The south facing facade of the Low Energy House.
A small house--the Low Energy House--was built in Hattula in southern Finland. Figure 1 presents the south facing facade. The house is conventional except the good insulation and the heating system. The floor area is about 110 m 2. The performance and the energy consumption of the heating system was studied from 1 January 1983 to 31 May 1984. During the measurements a family (parents and two children) lived in the house. At first there were some problems with the heating system, but from Summer 1983 the system worked quite well.
Fig. 2. The window screens.
375
A Finnish low energy house ENVELOPE
OF
THE
HOUSE
According to the Finnish building regulations the overall heat transfer coefficients of the exterior walls (U-values) must be 0.23-0.29 W K -] m -2 or less. The windows must have triple p~ne glazing, where the U-value is about 2W K -~ m -2. In the Low Energy House the U-values are over 50~ less than the regulations demand (Table 1). Table 1. The overall heat transfer coefficients (U-values) of exterior walls according to the regulations and in the Low Energy House U-value, W K - I m -2
Ceiling Floor Exterior wall Window Window with screens
Low Energy House
Regulations
0.10 0.40* 0.12 1.! 0.6
0.23 1.0* 0.29 2.1 --
*The values do not include the resistance of the earth.
The insulation material in the walls is polyurethane, which has a very lob conductivity, about 0.02 W K -~ m -~. Thus the wall structure doesn't become too thick, although the U-value is low. Compared to mineral wool, which has a conductivity of 0.04 W K -~ m -I, the wall structure is about 50~ thinner. The windows have 4-pane glazing and a selective coating in one glass. The U-value is 1.1 W K -~ m -2. In addition there is a movable screen between the two outermost panes (Fig. 2). The U-value, when the screens are down, is about 0.6 WK -~ m -2.
HEATING SYSTEM Figure 3 presents the principles of the heating system: The heat pump extracts heat from exhaust air and feeds it to the warm water circulation. If there is space heating demand, the warm water is circulated through the heating coil, otherwise through the water storage. A throttling valve controls the water circulation. Electricity is the energy source of the heat pump. The thermosyphon extracts heat from warm sewage water and feeds it to the fresh ventilation air. Heat is distributed into spaces with warm supply air, which includes about 70~ circulation air and 30~ fresh air (outdoor air). The supply air is distributed into rooms in air ducts below the floor. The air inlet apertures are below the windows. Fresh air in summer
Condenser / Fresh air in [-'1 II winter
^ ~
T o p water storage wfth
Exhaust air
II-'°
,.
eLectricaL heater (6St)
]
eLectricaL
ELe~rleoL ~ ~ heater cl~uGt.mL/-~ i~ i ~2hWl
pump WW÷SOeC*"~-
--*
Water ~torcKle(50Oil
T
~
/
I Evaporator
Fig. 3. The principles of the heating system.
stor~
376
TzMo K x t , m ~
et al.
The system has two water storages. The purpose is to keep the temperature of the water flowing to the heat pump condenser low and thus the coefficient of performance (COP) high. In wintertime the temperature of the bigger storage is about 40°C, which is enough for space heating. In summer, when there is no space heating demand, the temperature of the water storage will be kept higher. The warm tap water temperature must be at least 50°C for hygienic reasons. Therefore there is a smaller storage with a resistance heater to heat the tap water from 40 to 50°C. The heating effect of the heat pump is not big enough to meet the total heat demand in cold weather. Therefore there is a resistance heater in the supply air duct. The principle is, that the heat pump meets the basic load and the resistance heaters the peak load. In the bigger storage there are resistance heaters as a reserve energy source, if the heat pump should break. HEAT PUMP The heat pump is designed to cool the exhaust air from 21°C to about - 5 ° C . The exhaust air flow (I?o) is continuously about 0.03 m3s -I, which corresponds an air change rate of 0.5 times/hour. The heat effect obtained from the exhaust air, or the evaporator effect (~b,), is (1)
d~ = peeI2~=(h,. - he=)
where p~= is exhaust air density (1.25 kgm -3 at 10°C), h~, is indoor air enthalpy and he= is exhaust air enthalpy after the evaporator. Figure 4 presents the cooling process in enthalpy-humidity diagram. The relative humidity of indoor air is in wintertime about 35% and that of the exhaust air after the heat pump about 100%. With these values the enthalpy difference is h~ - h,o = 34 kJ kg- I. Thus the heating effect obtained from the exhaust air is 1.3 kW. This effect is obtainable continuously, except when the evaporator is defrosted. ,\
z5
~,
///,/~,,
/ x.\ \- //F / / ~'Y/
20
_,2L/o \
\
x
,.o
\\
,
,
5
tO
15
Humidity ratio ( g / k g )
Fig. 4. The coating of the exhaust air in enthalpy--humidity---diagram.
When the compressor is stopped, the warm exhaust air defrosts the evaporator. The defrosting process starts with a clock about once in two hours and stops, when the temperature of the outer surface exceeds + 10°C. The maximum defrosting time is, however, 10 min. During the defrosting the exhaust air flow is increased about 50% to enhance the process. The heat pump condenser effect (~bc) is approximately q~¢ =
8 e--1
q~,
(2)
A Finnish low energy house
377
where e is coefficient of performance (COP). When the COP is 8 = 2.5 and the evaporator effect q~, = 1.3 kW, the condenser effect is Oc = 2.2 kW. Thus the heat pump condenser may produce about 180 MJ daily. The compressor is a rotary sliding vane compressor, which is suitable in dwellings because of low noise. R22 is used as refrigerant. The evaporator is a copper tube aluminium fin heat exchanger and the condenser a coaxial heat exchanger. THERMOSYPHON The evaporator is a copper tube (25/27 ram) with copper fins on the outer surface. It is placed in a 5001 storage in the basement. The sewage waters, except WC-waters, are flowing to the storage and from there to the sewage system. The storage has a partition wall. The water flows at first to the left part, and from there over the partition wall to the right part. Under the partition wall there are small openings for water circulation. The purpose of the partition wall is to get a high temperature in the right part, where the evaporator is. The partition wall prevents the incoming water flow destroying the temperature stratification. The condenser is a copper tube aluminium pin construction (Fig. 5). It isplaced in the attic in the fresh air duct. The thermal conductance/length is large (about 30--40 W (K- ~m- ~) depending on the air velocity), and thus the temperature difference between the working fluid and air is low. The working fluid is R11.
Fig. 5. The pin pipe condenser in the thermosyphon. MEASUREMENTS The measurements were performed during 1 January 1983-31 May 1984. Among other things the following energies and temperatures were continuously measured: total (heating + household) electricity consumption electricity consumption of compressor and condenser pump electricity consumption of heating coil pump and fans electricity consumption of resistance heaters electricity consumption of sauna (Finnish bath) energy got from heat pump condenser
378
TiMo l ~ e t a l .
energy supplied to space heating (with water/air heating coil) warm water consumption about 20 temperatures in the building and heating system. The temperatures were rc~orded with copper--constantane thermoelements and two plotters, e.g. the temperatures of the exhaust air before and after the heat pump evaporator, the temperatures of the fresh ventilation air before and after the thermosyphon condenser, the sewage water temperatures before and after the storage and the indoor and outdoor air temperatures were measured. The measurement results are presented for the heating season 83/84 (1 September 1983-31 May 1984). There were some problems with the heat pump and the thermosyphon at first, and therefore the results of the early part of the year 1983 are not presented. The annual energy consumption is estimated with the aid of the resul{s of summer 1983. TOTAL ENERGY C O N S U M P T I O N Table 2 and Fig. 6 present the electricity consumption during the heating season 83/84. The total electricity consumption is 45 G J, where the share of the heating electricity is 26 GJ and that of the household consumption including the sauna 19 GJ. The energy got from the heat pump condenser is 32 GJ and that supplied with the water/air heating coil to space heating 30 GJ. The warm tap water consumption is 48 m 3. When the water is heated from 5 to 50°C, it takes about 9 GJ. Table 3 presents the energy balance of the water circle. The production includes the energy supplied with electrical heater to supply air, and therefore the production is a little higher than the consumption. 2500 !'""!
HousehoLd Sauna
Fans ÷ circulation pump Compreslor
20OO
+ condenser
pump
EtectrtcoL hooters ......
6
! .....
!.-,-!
:
GJ
i
Q C Q
4
I000
g t.
g LU
500
9
I0
II
12
i I
2
3
4
5
Month
!'o FI v
I0
FI
u
; ~" -5 2 ~ -~o
<[ ~ -I .'5
1
I0
I II
U
U
U
I I
I 2
I 3
I 12
Month
Fig. 6. The monthly electricity consumptions.
nil I 4
I 5
A Finnish low energy house
379
Table 2. The electricity consumption of the Low Energy House during 1 September 1983-31 May 1984
Electricity Compressor + condenser pump Electrical heaters Fans + heating coil pump Sauna (Finnish bath) Household Total electricity
GJ 12.2 8.3 5.4 3.6 16.2 45.0
The electricity consumption of heating is in summer (1 June--31 August) about 1.5 GJ. Electricity is needed in the heat pump for tap water heating and in the fans, but there is no space heating demand. When this is counted, the annual electricity consumption of heating is about 27.5 GJ. A conventional small house takes heating energy 60-70 GJ annually. Thus the energy consumption in the Low Energy House is over 50% less than in a conventional house. The reduction in energy costs is, however, not always so big, because in the Low Energy House electricity must be used, which is more expensive than e.g. district heating. Table 3. The energy balance of water circle Heating energy Heating energy produced GJ consumed Heat pump condenser 31.7 Warm air heating Resistance heaters* 8.3 Warm tap water Totally 40.0 Totally *Includes also the air heating coil.
GJ 29.5 9.0 38.5
HEAT PUMP P E R F O R M A N C E The coefficient of performance (s) for the heat pump is calculated from equation
Qc Ecp
8 ffi - -
(3)
where Q~ is condenser energy and E,p electricity consumption of compressor and condenser pump. Tables 2 and 3 and equation (3) give t -- 2.6. The COP excluding the electricity consumption of condenser pump is about 2.8.
2o 15
1
co__
_
/
A
Air before evaporator
pressar starts
v
--
Air after evaporator
I/ I 5 Timo ( h )
Fig. 7. The exhaust air temperature during the defrost cycle. 5 April 1984, when the outdoor temperature is about - 3°C.
380
Tiuo ~
et al.
The air flow through the evaporator is due to the exhaust air fan. The evaporator causes an extra pressure drop and therefore need for fan energy. However, the pressure drop is so little in the evaporator used, that the increase of fan energy may neglected in the COP. In another Finnish study [3] for an exhaust air heat pump the value e = 3.5 was measured. During the measurements the indoor air temperature was about 21°C. The evaporator cooled the exhaust air to - 3 - - 4 ° C . Figure 7 presents the defrosting process in the evaporator. It is accomplished with a clock about once in two hours. The defrosting takes about 10 min. After the defrosting has ended it takes about 10 min to reach the stationary condition. The defrosting influences to a certain degree on the heat pump performance, because the relative time, when the compressor is out of stationary conditions, is in Fig. 7 about 20%. THERMOSYPHON
PERFORMANCE
Because the sewage water flow is irregular, it is difficult to measure the energy obtained from it. The heating effect obtained from the condenser is, however, quite regular. It depends strongly from the outdoor air temperature. The greater the temperature difference between the condensing and evaporating temperatures is the greater the heating effect is. The water temperature in storage doesn't change very much, and therefore the outdoor air temperature influences strongly on the effect. Figure 8 presents the performance of the thermosyphon during three days in December. The outdoor air temperature is 0-15°C. The temperature of the inlet air to condenser is a little higher, because it is extracted from the attic. The heat losses of the roof heat the air. Figure 9 presents the condenser effect and the temperature difference in air as a function of outdoor air temperature. The condenser effect (~bc,,) is calculated from the equation dPc,, = PoCa1~'~ To
(4)
where po is density of air (1.27 kg m -3 at 5°C), co is specific heat of air (1.0 kJ kg -I K-t), I2 is air flow rate (0.032 m 3 s -I) and AT, is temperature difference in air. The average outdoor temperature during the heating season is about 0°C. The corresponding temperature difference is 3.2 K (Fig. 9). Equation (4) gives the average condensing effect O,h = 130 W. When the length of the heating season is about 6500 h, the corresponding condenser energy is Q,, = 3.1 GJ.
} f Io
~ /
/
~
Sewage water on t h e s u r f o c e
~
/
M
J
side
c
Fresh air after
-,o[, , o,o, Hour
Fig. 8. The temperatures in the sewage water heat recovery system (thermosyphon) during three days (9-11 December).
A Finnish low energy house
381 400
I0
8
320
%%
.?-.
:
24o
.%%
g
• . s'~,,,. 160
4 o
•
E z
0
I -15
-I0
-5
%%%~e • %%
0
5
8O
JO
u, c o "r
15
InLet. temperature (oC)
Fig. 9. The condenser effect and the temperature difference of air in the thermosyphon. Air flow 0.032m3s- t. The efficiency of the thermosyphon may be calculated from the equation ,7,h =
Qcth Qtw
(5)
where Qc~ is condenser energy and Q~ is energy used to heat tap water. With the values Qc,~= 3.1 GJ and Q~ = 9.0 GJ, equation (5) gives ~/,, = 0.34. Thus the average efficiency of the thermosyphon is about 30% during the heating season. In the summer the efficiency is very low because of the high outdoor air temperatures. Also in the summer very little heating energy is needed. EXPERIENCES There were some problems in the begining with the heating system, e.g. there were leakages in the thermosyphon and air ducts, but they were repaired easily. Also there were problems with the control system. The heat pump worked in too short periods, because the electrical heaters in the water storage were coupled on too early. The system was changed so that the electrical heaters were taken out of use. They were left for a reserve energy source, if the heat pump should break. Instead, an electrical heater for supply air and a little water storage for tap water was installed. This change had the principal advantage, that the temperature of the bigger storage, where the heat pump feeds energy, decreased and thus lowered the temperature difference between the condenser and evaporator. After these repairs and changes had been done, the system worked without problems. A positive thing was, that the evaporator of the thermosyphon hardly got dirty. The dirt on the surface could be removed easily. COSTS The Low Energy House is compared to a conventional small house, where electricity is the energy source and water radiators the heat distribution system. The insulation of exterior walls is according to the building regulations. The comparison could be made also with district heating, where the energy price is lower, but the capital costs, including the fees for joining, are higher. The result would be however about the same. The capital costs of the Low Energy House are higher than in a conventional house because of the better insulation and the energy saving heating system. The additional costs of the insulation are about 2700 USD and that of the heating system 3300 USD (Table 4). The price of heating electricity in Finland is quite low, about 11 USD/GJ. The annual energy saving obtained with the Low Energy House is 30-40 GJ. The saving in the energy costs is thus about 330-440 USD. When this is compared to the capital costs, the pay-back time without interest is about 15 yr. With the 5% interest the pay-back time is over 30 yr. The conclusion is that the price of electricity is too low, that the Low Energy House would be profitable now.
382
TIMo KAU~MAet aL Table 4. The additional costs of the Low Energy house USD Thermosyphon 1300 Heat pump 2000 Additional insulation 2700 Totally 6000
The economics of the thermosyphon and heat pump may be estimated in a conventional small house, which takes 60 GJ. Here the share of the tap water heating is about 15 GJ and that of the space heating 45 GJ. The energy saving of the thermosyphon is about 4 GJ and the capital costs 1300USD. Even without interest the pay-back time is very long, about 30yr. Thus the thermosyphon is clearly uneconomicalin a small house, where the tap water consumption is small. The economics would be better, when the tap water consumption is high, e.g. in apartment houses, swimming halls, laundries etc. The energy saving of the exhaust air heat pump may be estimated to about 25 GJ. In the Low Energy House the energy saving was 19.5 GJ (Tables 2 and 3). In a conventional house the compressor working hours are bigger and thus the energy saving is bigger. The capital costs are about 2000 USD. The pay-back time with the 5~ interest is about 10 yr. Thus the exhaust air heat pump is a quite economical energy saving device (Table 5). Table 5. The energy saving of the heat pump and the thermosypbon in a conventional house Pay-back time with Energy saving 5% interest GJ yr - * yr Heat pump 25 10 Thermosyphon 4 --
SUMMARY The energy consumption of the Low Energy House has been studied. The energy saving means are the good insulation of building envelope and heat recovery from exhaust air with a heat pump and from sewage water with a thermosyphon. The annual energy consumption of heating is about 27 GJ or 250 MJ floor--m -2. The energy saving compared to a conventional small house is over 509/0, about 30-40 GJ. The capital costs of the Low Energy House are about 6000 USD higher than in a conventional house. When the mean price of heating electricity is 11 USD/GJ, the reduction in heating costs is about 390 USD annually. With the 59/0 interest the pay-back time is over 30 yr. Thus the energy price is now too cheap, that the Low energy House would be profitable, although it saves considerably energy. There are too many energy saving means in one house. Also, the heat recovery from sewage waters is clearly uneconomical in small houses, where the annual tap water consumption is small. The exhaust air heat pump installed in a conventional house is quite economical. The energy saving obtainable is about 25 GJ and its cost compared to a conventional heating system about 2000 USD higher. With 5~ interest the pay-back time is about 10 yr. REFERENCES 1. T. Kalema, J. M ~ t t a and J. Palm, Three solar houses--the measurement o f energy consumption. Espoo 1983. Technical Research Centre o f Finland. Report 186. 86 p. (in Finnish). 2. T. Kalema and J. Palm, A small house with solar and heat pump heating system--the measurement o f energy consumption. Espoo 1984. Technical Research Centre o f Finland. Report 287. 55 p. (in Finnish). 3. R. Wiksten, Measurement results of small house heat pumps. Espoo 1982, Technical Research Centre o f Finland. Report 92. 65 p. (in Finnish).
0198-7593/85$3.00+ .00 Pergamon Press Lid
A FINNISH LOW ENERGY HOUSE TIMO KALEMA Technical Research Centre of Finland, Laboratory of Heating and Ventilating, 02150 Espoo, Finland JAAKKO KARVINEN Makrotalo Ltd, Kornetintie 4, 00380 Heisinki, Finland and RaSTO CASTR~q Retermia Ltd, Kontiokatu 2 D, 15950 Lahti, Finland
AIm~'t--In the Finnish climate the annual heating energy demand in a modern small house, where the floor area is about 120m2, is 60-70 GJ. The energy includes both the space and the tap water heating. With a new heating system in an extremely well insulated house the annual energy consuAnption is 27 GJ. The heating system includes heat recovery from exhaust air with a heat pump and from sewage water with a thermosyphon. The heating costs are very low. When the mean price of electricity is 11 USD/GJ, the annual heating costs (without capital costs) are about 300 USD.
NOMENCLATURE c E h Q T I~ U e
specificheat [J ks- l K-'] electricalenergy [J] enthalpy [J kg- l] heat energy [J] temperature [K, °C] air volume flow [m3S-I] over-all heat transfer coefficient [W K -~ m -2] coeffident of performance efficiency p air density [kg m-3] heat effect [W] A difference COP coefficient of performance Subscripts
a c cp cth
e ea ia th
air heat pump condenser compressor thermosyphon condenser heat pump evaporator exhaust air indoor air thermosyphon INTRODUCTION
The energy c o n s u m p t i o n o f buildings m a y be decreased with a g o o d insulation o f building envelope and with heat recovery f r o m exhaust air and f r o m sewage water. T h e Finnish building c o m p a n y M a k r o t a l o Ltd has developed with Technical Research Centre o f Finland and Retermia Ltd a low energy house, where these principles are used. The p u r p o s e was to develop a small house, where the energy c o n s u m p t i o n is low without bargaining o f the living quality (ventilation, w a r m water c o n s u m p t i o n and i n d o o r air temperature). The house should be such, that it could be placed everywhere. This d e m a n d m e a n t that the earth heat p u m p could not be used, because the earth pipes take quite a big area. Also solar energy could not be used, because the Finnish climate is such, that small solar heating systems w o r k very p o o r l y [1, 2]. The best energy saving m e t h o d s seemed to be the g o o d insulation o f building envelope and the heat recovery f r o m exhaust air and sewage water. 373
374
TXMO KAt.EMA et al.
j
Fig. 1. The south facing facade of the Low Energy House.
A small house--the Low Energy House--was built in Hattula in southern Finland. Figure 1 presents the south facing facade. The house is conventional except the good insulation and the heating system. The floor area is about 110 m 2. The performance and the energy consumption of the heating system was studied from 1 January 1983 to 31 May 1984. During the measurements a family (parents and two children) lived in the house. At first there were some problems with the heating system, but from Summer 1983 the system worked quite well.
Fig. 2. The window screens.
375
A Finnish low energy house ENVELOPE
OF
THE
HOUSE
According to the Finnish building regulations the overall heat transfer coefficients of the exterior walls (U-values) must be 0.23-0.29 W K -] m -2 or less. The windows must have triple p~ne glazing, where the U-value is about 2W K -~ m -2. In the Low Energy House the U-values are over 50~ less than the regulations demand (Table 1). Table 1. The overall heat transfer coefficients (U-values) of exterior walls according to the regulations and in the Low Energy House U-value, W K - I m -2
Ceiling Floor Exterior wall Window Window with screens
Low Energy House
Regulations
0.10 0.40* 0.12 1.! 0.6
0.23 1.0* 0.29 2.1 --
*The values do not include the resistance of the earth.
The insulation material in the walls is polyurethane, which has a very lob conductivity, about 0.02 W K -~ m -~. Thus the wall structure doesn't become too thick, although the U-value is low. Compared to mineral wool, which has a conductivity of 0.04 W K -~ m -I, the wall structure is about 50~ thinner. The windows have 4-pane glazing and a selective coating in one glass. The U-value is 1.1 W K -~ m -2. In addition there is a movable screen between the two outermost panes (Fig. 2). The U-value, when the screens are down, is about 0.6 WK -~ m -2.
HEATING SYSTEM Figure 3 presents the principles of the heating system: The heat pump extracts heat from exhaust air and feeds it to the warm water circulation. If there is space heating demand, the warm water is circulated through the heating coil, otherwise through the water storage. A throttling valve controls the water circulation. Electricity is the energy source of the heat pump. The thermosyphon extracts heat from warm sewage water and feeds it to the fresh ventilation air. Heat is distributed into spaces with warm supply air, which includes about 70~ circulation air and 30~ fresh air (outdoor air). The supply air is distributed into rooms in air ducts below the floor. The air inlet apertures are below the windows. Fresh air in summer
Condenser / Fresh air in [-'1 II winter
^ ~
T o p water storage wfth
Exhaust air
II-'°
,.
eLectricaL heater (6St)
]
eLectricaL
ELe~rleoL ~ ~ heater cl~uGt.mL/-~ i~ i ~2hWl
pump WW÷SOeC*"~-
--*
Water ~torcKle(50Oil
T
~
/
I Evaporator
Fig. 3. The principles of the heating system.
stor~
376
TzMo K x t , m ~
et al.
The system has two water storages. The purpose is to keep the temperature of the water flowing to the heat pump condenser low and thus the coefficient of performance (COP) high. In wintertime the temperature of the bigger storage is about 40°C, which is enough for space heating. In summer, when there is no space heating demand, the temperature of the water storage will be kept higher. The warm tap water temperature must be at least 50°C for hygienic reasons. Therefore there is a smaller storage with a resistance heater to heat the tap water from 40 to 50°C. The heating effect of the heat pump is not big enough to meet the total heat demand in cold weather. Therefore there is a resistance heater in the supply air duct. The principle is, that the heat pump meets the basic load and the resistance heaters the peak load. In the bigger storage there are resistance heaters as a reserve energy source, if the heat pump should break. HEAT PUMP The heat pump is designed to cool the exhaust air from 21°C to about - 5 ° C . The exhaust air flow (I?o) is continuously about 0.03 m3s -I, which corresponds an air change rate of 0.5 times/hour. The heat effect obtained from the exhaust air, or the evaporator effect (~b,), is (1)
d~ = peeI2~=(h,. - he=)
where p~= is exhaust air density (1.25 kgm -3 at 10°C), h~, is indoor air enthalpy and he= is exhaust air enthalpy after the evaporator. Figure 4 presents the cooling process in enthalpy-humidity diagram. The relative humidity of indoor air is in wintertime about 35% and that of the exhaust air after the heat pump about 100%. With these values the enthalpy difference is h~ - h,o = 34 kJ kg- I. Thus the heating effect obtained from the exhaust air is 1.3 kW. This effect is obtainable continuously, except when the evaporator is defrosted. ,\
z5
~,
///,/~,,
/ x.\ \- //F / / ~'Y/
20
_,2L/o \
\
x
,.o
\\
,
,
5
tO
15
Humidity ratio ( g / k g )
Fig. 4. The coating of the exhaust air in enthalpy--humidity---diagram.
When the compressor is stopped, the warm exhaust air defrosts the evaporator. The defrosting process starts with a clock about once in two hours and stops, when the temperature of the outer surface exceeds + 10°C. The maximum defrosting time is, however, 10 min. During the defrosting the exhaust air flow is increased about 50% to enhance the process. The heat pump condenser effect (~bc) is approximately q~¢ =
8 e--1
q~,
(2)
A Finnish low energy house
377
where e is coefficient of performance (COP). When the COP is 8 = 2.5 and the evaporator effect q~, = 1.3 kW, the condenser effect is Oc = 2.2 kW. Thus the heat pump condenser may produce about 180 MJ daily. The compressor is a rotary sliding vane compressor, which is suitable in dwellings because of low noise. R22 is used as refrigerant. The evaporator is a copper tube aluminium fin heat exchanger and the condenser a coaxial heat exchanger. THERMOSYPHON The evaporator is a copper tube (25/27 ram) with copper fins on the outer surface. It is placed in a 5001 storage in the basement. The sewage waters, except WC-waters, are flowing to the storage and from there to the sewage system. The storage has a partition wall. The water flows at first to the left part, and from there over the partition wall to the right part. Under the partition wall there are small openings for water circulation. The purpose of the partition wall is to get a high temperature in the right part, where the evaporator is. The partition wall prevents the incoming water flow destroying the temperature stratification. The condenser is a copper tube aluminium pin construction (Fig. 5). It isplaced in the attic in the fresh air duct. The thermal conductance/length is large (about 30--40 W (K- ~m- ~) depending on the air velocity), and thus the temperature difference between the working fluid and air is low. The working fluid is R11.
Fig. 5. The pin pipe condenser in the thermosyphon. MEASUREMENTS The measurements were performed during 1 January 1983-31 May 1984. Among other things the following energies and temperatures were continuously measured: total (heating + household) electricity consumption electricity consumption of compressor and condenser pump electricity consumption of heating coil pump and fans electricity consumption of resistance heaters electricity consumption of sauna (Finnish bath) energy got from heat pump condenser
378
TiMo l ~ e t a l .
energy supplied to space heating (with water/air heating coil) warm water consumption about 20 temperatures in the building and heating system. The temperatures were rc~orded with copper--constantane thermoelements and two plotters, e.g. the temperatures of the exhaust air before and after the heat pump evaporator, the temperatures of the fresh ventilation air before and after the thermosyphon condenser, the sewage water temperatures before and after the storage and the indoor and outdoor air temperatures were measured. The measurement results are presented for the heating season 83/84 (1 September 1983-31 May 1984). There were some problems with the heat pump and the thermosyphon at first, and therefore the results of the early part of the year 1983 are not presented. The annual energy consumption is estimated with the aid of the resul{s of summer 1983. TOTAL ENERGY C O N S U M P T I O N Table 2 and Fig. 6 present the electricity consumption during the heating season 83/84. The total electricity consumption is 45 G J, where the share of the heating electricity is 26 GJ and that of the household consumption including the sauna 19 GJ. The energy got from the heat pump condenser is 32 GJ and that supplied with the water/air heating coil to space heating 30 GJ. The warm tap water consumption is 48 m 3. When the water is heated from 5 to 50°C, it takes about 9 GJ. Table 3 presents the energy balance of the water circle. The production includes the energy supplied with electrical heater to supply air, and therefore the production is a little higher than the consumption. 2500 !'""!
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Fig. 6. The monthly electricity consumptions.
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A Finnish low energy house
379
Table 2. The electricity consumption of the Low Energy House during 1 September 1983-31 May 1984
Electricity Compressor + condenser pump Electrical heaters Fans + heating coil pump Sauna (Finnish bath) Household Total electricity
GJ 12.2 8.3 5.4 3.6 16.2 45.0
The electricity consumption of heating is in summer (1 June--31 August) about 1.5 GJ. Electricity is needed in the heat pump for tap water heating and in the fans, but there is no space heating demand. When this is counted, the annual electricity consumption of heating is about 27.5 GJ. A conventional small house takes heating energy 60-70 GJ annually. Thus the energy consumption in the Low Energy House is over 50% less than in a conventional house. The reduction in energy costs is, however, not always so big, because in the Low Energy House electricity must be used, which is more expensive than e.g. district heating. Table 3. The energy balance of water circle Heating energy Heating energy produced GJ consumed Heat pump condenser 31.7 Warm air heating Resistance heaters* 8.3 Warm tap water Totally 40.0 Totally *Includes also the air heating coil.
GJ 29.5 9.0 38.5
HEAT PUMP P E R F O R M A N C E The coefficient of performance (s) for the heat pump is calculated from equation
Qc Ecp
8 ffi - -
(3)
where Q~ is condenser energy and E,p electricity consumption of compressor and condenser pump. Tables 2 and 3 and equation (3) give t -- 2.6. The COP excluding the electricity consumption of condenser pump is about 2.8.
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1
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_
/
A
Air before evaporator
pressar starts
v
--
Air after evaporator
I/ I 5 Timo ( h )
Fig. 7. The exhaust air temperature during the defrost cycle. 5 April 1984, when the outdoor temperature is about - 3°C.
380
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The air flow through the evaporator is due to the exhaust air fan. The evaporator causes an extra pressure drop and therefore need for fan energy. However, the pressure drop is so little in the evaporator used, that the increase of fan energy may neglected in the COP. In another Finnish study [3] for an exhaust air heat pump the value e = 3.5 was measured. During the measurements the indoor air temperature was about 21°C. The evaporator cooled the exhaust air to - 3 - - 4 ° C . Figure 7 presents the defrosting process in the evaporator. It is accomplished with a clock about once in two hours. The defrosting takes about 10 min. After the defrosting has ended it takes about 10 min to reach the stationary condition. The defrosting influences to a certain degree on the heat pump performance, because the relative time, when the compressor is out of stationary conditions, is in Fig. 7 about 20%. THERMOSYPHON
PERFORMANCE
Because the sewage water flow is irregular, it is difficult to measure the energy obtained from it. The heating effect obtained from the condenser is, however, quite regular. It depends strongly from the outdoor air temperature. The greater the temperature difference between the condensing and evaporating temperatures is the greater the heating effect is. The water temperature in storage doesn't change very much, and therefore the outdoor air temperature influences strongly on the effect. Figure 8 presents the performance of the thermosyphon during three days in December. The outdoor air temperature is 0-15°C. The temperature of the inlet air to condenser is a little higher, because it is extracted from the attic. The heat losses of the roof heat the air. Figure 9 presents the condenser effect and the temperature difference in air as a function of outdoor air temperature. The condenser effect (~bc,,) is calculated from the equation dPc,, = PoCa1~'~ To
(4)
where po is density of air (1.27 kg m -3 at 5°C), co is specific heat of air (1.0 kJ kg -I K-t), I2 is air flow rate (0.032 m 3 s -I) and AT, is temperature difference in air. The average outdoor temperature during the heating season is about 0°C. The corresponding temperature difference is 3.2 K (Fig. 9). Equation (4) gives the average condensing effect O,h = 130 W. When the length of the heating season is about 6500 h, the corresponding condenser energy is Q,, = 3.1 GJ.
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~ /
/
~
Sewage water on t h e s u r f o c e
~
/
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J
side
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Fresh air after
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Fig. 8. The temperatures in the sewage water heat recovery system (thermosyphon) during three days (9-11 December).
A Finnish low energy house
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Fig. 9. The condenser effect and the temperature difference of air in the thermosyphon. Air flow 0.032m3s- t. The efficiency of the thermosyphon may be calculated from the equation ,7,h =
Qcth Qtw
(5)
where Qc~ is condenser energy and Q~ is energy used to heat tap water. With the values Qc,~= 3.1 GJ and Q~ = 9.0 GJ, equation (5) gives ~/,, = 0.34. Thus the average efficiency of the thermosyphon is about 30% during the heating season. In the summer the efficiency is very low because of the high outdoor air temperatures. Also in the summer very little heating energy is needed. EXPERIENCES There were some problems in the begining with the heating system, e.g. there were leakages in the thermosyphon and air ducts, but they were repaired easily. Also there were problems with the control system. The heat pump worked in too short periods, because the electrical heaters in the water storage were coupled on too early. The system was changed so that the electrical heaters were taken out of use. They were left for a reserve energy source, if the heat pump should break. Instead, an electrical heater for supply air and a little water storage for tap water was installed. This change had the principal advantage, that the temperature of the bigger storage, where the heat pump feeds energy, decreased and thus lowered the temperature difference between the condenser and evaporator. After these repairs and changes had been done, the system worked without problems. A positive thing was, that the evaporator of the thermosyphon hardly got dirty. The dirt on the surface could be removed easily. COSTS The Low Energy House is compared to a conventional small house, where electricity is the energy source and water radiators the heat distribution system. The insulation of exterior walls is according to the building regulations. The comparison could be made also with district heating, where the energy price is lower, but the capital costs, including the fees for joining, are higher. The result would be however about the same. The capital costs of the Low Energy House are higher than in a conventional house because of the better insulation and the energy saving heating system. The additional costs of the insulation are about 2700 USD and that of the heating system 3300 USD (Table 4). The price of heating electricity in Finland is quite low, about 11 USD/GJ. The annual energy saving obtained with the Low Energy House is 30-40 GJ. The saving in the energy costs is thus about 330-440 USD. When this is compared to the capital costs, the pay-back time without interest is about 15 yr. With the 5% interest the pay-back time is over 30 yr. The conclusion is that the price of electricity is too low, that the Low Energy House would be profitable now.
382
TIMo KAU~MAet aL Table 4. The additional costs of the Low Energy house USD Thermosyphon 1300 Heat pump 2000 Additional insulation 2700 Totally 6000
The economics of the thermosyphon and heat pump may be estimated in a conventional small house, which takes 60 GJ. Here the share of the tap water heating is about 15 GJ and that of the space heating 45 GJ. The energy saving of the thermosyphon is about 4 GJ and the capital costs 1300USD. Even without interest the pay-back time is very long, about 30yr. Thus the thermosyphon is clearly uneconomicalin a small house, where the tap water consumption is small. The economics would be better, when the tap water consumption is high, e.g. in apartment houses, swimming halls, laundries etc. The energy saving of the exhaust air heat pump may be estimated to about 25 GJ. In the Low Energy House the energy saving was 19.5 GJ (Tables 2 and 3). In a conventional house the compressor working hours are bigger and thus the energy saving is bigger. The capital costs are about 2000 USD. The pay-back time with the 5~ interest is about 10 yr. Thus the exhaust air heat pump is a quite economical energy saving device (Table 5). Table 5. The energy saving of the heat pump and the thermosypbon in a conventional house Pay-back time with Energy saving 5% interest GJ yr - * yr Heat pump 25 10 Thermosyphon 4 --
SUMMARY The energy consumption of the Low Energy House has been studied. The energy saving means are the good insulation of building envelope and heat recovery from exhaust air with a heat pump and from sewage water with a thermosyphon. The annual energy consumption of heating is about 27 GJ or 250 MJ floor--m -2. The energy saving compared to a conventional small house is over 509/0, about 30-40 GJ. The capital costs of the Low Energy House are about 6000 USD higher than in a conventional house. When the mean price of heating electricity is 11 USD/GJ, the reduction in heating costs is about 390 USD annually. With the 59/0 interest the pay-back time is over 30 yr. Thus the energy price is now too cheap, that the Low energy House would be profitable, although it saves considerably energy. There are too many energy saving means in one house. Also, the heat recovery from sewage waters is clearly uneconomical in small houses, where the annual tap water consumption is small. The exhaust air heat pump installed in a conventional house is quite economical. The energy saving obtainable is about 25 GJ and its cost compared to a conventional heating system about 2000 USD higher. With 5~ interest the pay-back time is about 10 yr. REFERENCES 1. T. Kalema, J. M ~ t t a and J. Palm, Three solar houses--the measurement o f energy consumption. Espoo 1983. Technical Research Centre o f Finland. Report 186. 86 p. (in Finnish). 2. T. Kalema and J. Palm, A small house with solar and heat pump heating system--the measurement o f energy consumption. Espoo 1984. Technical Research Centre o f Finland. Report 287. 55 p. (in Finnish). 3. R. Wiksten, Measurement results of small house heat pumps. Espoo 1982, Technical Research Centre o f Finland. Report 92. 65 p. (in Finnish).