Single family house in the Cold Climate Renewable Energy Strategy

Tobias Bostrom and Johan Smeds

Table 8.3.1 Targets for the single family house in the Cold Climate Renewable Energy Strategy

Targets

Space heating

25 kWh/m2a

Non-renewable primary energy:

(space heating + water heating +

electricity for mechanical systems)

60 kWh/m2a

This section presents a solution for the single family house in the cold climate. As a reference for the cold climate, the city of Stockholm is considered. The solution is focused on the use of renewable energy sources.

8.3.1 Solution 2: Renewable energy with solar combi-system and biomass or condensing gas boiler

Building envelope

The opaque part of the building envelope is a wooden lightweight frame with mineral wool as insulating material. The windows have a frame ratio of 30 per cent, triple glazing, one low-e coating and are filled with krypton gas. The U-values of the construction components are shown in Table 8.3.2. The detailed layers of the construction are shown in Table 8.3.8.

Table 8.3.2 Building component U-values for solution 2

Component

U-value (W/m2K)

Walls

0.14

Roof

0.15

Floor

0.20

Windows (frame + glass)

0.92

Window frame

1.20

Window glass

0.80

Whole building envelope

0.21

Mechanical systems

A balanced (mechanical) ventilation system with 80 per cent heat recovery and a bypass for summer ventilation is considered.

Heat is supplied by a solar combi-system with a biomass burner and 6 m2 of direct-flow vacuum tube collectors or 7.5 m2 of flat-plate collectors. The solar collectors are mounted at an angle of 40° and a storage tank of 0.5 m3 is assumed. The solar collector area is optimized for achieving 100% coverage of the heat demand during summer. The heat is distributed in the building by hot water radiant heating. An alternative to using the biomass boiler is a solar combi-system with a condensing gas boiler.

A question that could be posed is whether or not the solar collector system should be of a DHW or combi-type. The better a house is insulated, the less advantage is achieved from a combi-system since the heating season is greatly reduced. By using a DHW solar collector system instead of a combi-system, the overall performance of the solar system for this typical example solution will decrease by 2 per cent to 3 per cent. The combi-system does not achieve a higher overall efficiency and it costs more. On the other hand, a space heating distribution system is needed in any case. If this is achieved by a water-based system, the added cost for a combi-system is mainly just the additional collector area and the upsizing of the storage tank.

Energy performance

Space heating demand: the space heating demand was computed DEROB-LTH (Kvist, 2005). Figure 8.3.1 compares this high-performance building and a reference building according to current building standards. The heating season extends from November to March and the annual space heating demand is 3700 kWh. Other assumptions made for the DEROB simulations are:

• maximum room temperatures: 23°C during winter and 26°C during summer (assumes use of shading devices and window ventilation);

• heat recovery: 80% efficiency.

H reference case

Source: Tobias Bostrom and Johan Smeds

Figure 8.3.1 Space heating demand (annual total 3701 kWh/a)

Peak load of space heating system: the hourly heat loads of the heating system are calculated with DEROB-LTH. The simulation is performed without direct solar radiation in order to simulate a totally shaded building. A comparison of the peak loads for each month for this high-performance building and a reference building according to current building standards is shown in Figure 8.3.2. The peak load for the most extreme hour of the year is 2515 W and occurs in January. While the peak occurs in January, near peak loads also occur during some hours in February, March and December. Outside of these months, the peaks fall off very sharply.

Domestic hot water demand: the net DHW heat demand is approximately 3150 kWh/a or 21 kWh/m2a for two adults and two children who occupy a typical single family detached house. The DHW temperature is set to 55°C and the average DHW consumption per person is 40 litres per person per day. Consequently, the model single family house consumes 160 litres of domestic hot water per day. The average temperature over the year of cold tap water in Stockholm is 8.5°C. The temperature of the hot water was set to 50°C at the faucet. The on/off temperature set points for the thermostat in the tank were set to 55/57°C.

System losses: the system losses consist mainly of losses from the hot water storage tank, but also from pipe losses in the distribution system and conversion losses in the boiler. The system losses are dependent on several parameters and how the losses actually are defined. The losses provided by the used solar collector simulation programme are the tank losses, which include the total heat losses through the tank wall, base and cover, and the connection losses. The tank losses become larger with an increase in tank size and/or increase of solar collector area. The tank losses for a solar collector system with 7.5 m2 of collectors and a 600 litre tank are about 950 kWh per year or 6.3 kWh/m2a (per living area). The coefficient of performance (COP) for the biomass boiler is 85 per cent, resulting in conversion losses of 5.6 kWh/m2a. Due to the assumed COP of 100 per cent for a condensing gas boiler, the conversion losses are set to zero.

Household electricity: the amount of household electricity used by two adults and two children is approximately 2500 kWh or 16.6 kWh/m2a. The primary energy calculations shown in Tables 8.3.4 and 8.3.5 do not include household electricity since this factor can vary considerably, depending on the occupant's behaviour.

Total energy use: the total energy use of heat for DHW, space heating and system losses is 7800 kWh/a and the end energy use of electricity for household electricity and mechanical systems is approximately 3240 kWh/a (see Table 8.3.3).

Table 8.3.3 Total energy use for solution 2

Total energy use

kWh/m2a

kWh/a

Space heating

24.7

3700

DHW heating

21.0

3150

System losses

6.3

950

Electricity, mechanical systems

5.0

750

Household electricity

16.6

2490

Non-renewable primary energy demand and CO2 equivalent emissions: the conversion factors for the furnace are set to 0.85 for the pellet burner and 1.0 for the condensing gas burner. The factors for primary energy and CO2 emissions are taken from GEMIS (GEMIS, 2004). The amount of remaining energy for space heating, DHW, tank and pipe losses after taking solar gains into account is calculated in Polysun. The thermal solar combi-system consists of 7.5 m2 of flat-plate collectors tilted 40° and a 600 litre storage tank.

According to Tables 8.3.4 and 8.3.5, the use of non-renewable primary energy is 17 kWh/m2a for the solar combi-system with a biomass boiler and 47.9 kWh/m2a if a solar combi-system with condensing gas boiler is used. The CO2 equivalent emissions sum up to 3.8 kg/m2a for the solar combi-system with a biomass boiler and 10 kg/m2a if a solar combi-system with condensing gas boiler is used.

Table 8.3.4 Primary energy demand and CO2 emissions for solar combi-system with biomass boiler

Net Energy (kWh/m2a)

Total Energy Use (kWh/m'a)

Delivered energy (kWh/m2a)

Non renewable primary energy

CO2 equivalent emissions

Energy use

Energy source

factor (-)

(kWh/m2a)

factor (kg/kWh)

(kg/m!a)

Mechanical systems

5.0

Mechanical systems

5.0

Electricity

5.0

Electricity

5.0

2.35

11.8

0.43

2.2

Space heating

24.7

Space heating

24.7

Wood pellets

37.3

Wood pellets

37.3

0.14

5.2

0.04

1.6

DHW

21.0

DHW

21.0

Tank and circulation losses

6.3

Solar

20.3

Conversion losses

5.6

Total

50.7

62.6

62.6

42.3

17.0

3.8

Table 8.3.5 Primary energy demand and CO2 emissions for solar combi-system with condensing gas boiler

Table 8.3.5 Primary energy demand and CO2 emissions for solar combi-system with condensing gas boiler

Net Energy (kWh/m2a)

Total Energy Use (kWh/m2a)

Delivered energy (kWh/m2a)

Non renewable primary energy

CO2 equivalent emissions

Energy use

Energy source

factor (-)

(kWh/m2a)

factor (kg/kWh)

(kg/m2a)

Mechanical systems

5.0

Mechanical systems

5.0

Electricity

5.0

Electricity

5.0

2.35

11.8

0.43

2.2

Space heating

24.7

Space heating

24.7

Gas

31.7

Gas

31.7

1.14

36.1

0.25

7.8

DHW

21.0

DHW

21.0

Tank and circulation losses

6.3

Solar

20.3

Conversion losses

0.0

Total

50.7

57.0

57.0

36.7

47.9

10.0

8.3.2 Sensitivity analysis to key solar active parameters

The solar collector system has been studied and simulated extensively using the Swiss simulation programme Polysun (see Solartechnik Prüfung Forschnung, www.spf.ch). Pipe losses and storage losses in Polysun are calculated and taken into account as internal gains. The most important fixed parameters for the simulations are:

• Stockholm climate: Meteonorm generated (Meteotest, 2004);

• insulation levels: 150 mm for the tank and 25 mm for the piping;

• piping lengths: 3 m outdoors and 12 m indoors; and

• collector types: either a high-performing flat plate or an evacuated tube.

In the following sections we have chosen to show the solar collector system's sensitivity to a few key optimization parameters, such as:

• tilt angle 40° or 90° (roof or wall placement);

• collector type, evacuated tube or flat plate;

• hot water set temperature; and

Rather than presenting the energy conversion efficiency or solar fraction for the solar thermal collector systems, most figures show the remaining auxiliary demand needed to cover the DHW and space heating demands.

Collector orientation

The first question is: what happens if the collector is not facing directly towards south? The simulations show that the direction is not critical. The efficiency of the collector will be more than 95 per cent of maximum output as long as the orientation or azimuth direction is within +/-30° from south.

Collector area

Figure 8.3.3 shows how the auxiliary demand for a flat-plate solar combi-system varies during the summer months for collector areas between 5 m2 and 10 m2. When designing a conventional solar thermal collector system, one should try to obtain 100 per cent coverage during June to August. The demand during the summer only consists of DHW, which amounts to 262 kWh/month (excluding storage tank losses). It can be seen that 5 m2 is too small in area in order to get complete coverage during the summer months. If the size is increased to 10 m2, the auxiliary demand becomes almost non-existant from May to August and it is likely that the solar system will suffer from overheating problems during the summer.

Table 8.3.6 shows what happens with the solar collector output and the solar fraction when the area is increased from 5 m2 to 10 m2. The useful collector output drastically decreases when the area is increased. This is mainly due to the fact that the 5 m2 collector almost already covers the whole demand during the summer. A larger collector will produce non-useful waste heat during the summer, which can be harmful for the solar collector. The absorber itself can sustain periods with tempera-

■ 5 m2, 0.5 m3 tank, total auxiliary yearly demand 34.1 kWh/m2 0 7.5 m2, 0.6 m3 tank, total auxiliary yearly demand 31.8 kWh/mz □ 10 m5, 0,7 m3 tank, total auxiliary yearly demand 30.3 kWh/m*

Source: Tobias Boström and Johan Smeds

Apr May

Jul Aug

Figure 8.3.3 The auxiliary demand's dependence upon collector area during the summer months and the total auxiliary annual demand in kWh/m2 (living area)

Apr May

Jul Aug

Sep tures above 200°C; but the water/glycol mixture will start to disintegrate above about 140°C. The disintegration results in the fact that glycol lumps are formed inside the tubes, which eventually blocks, and the absorber will be unusable.

A collector of about 7.5 m2 is, however, needed if one wants to be completely independent of an auxiliary system during the summer. A 7.5 m2 system will, during one year, only get about 6 collector temperature spikes over 140°C, while 10 m2 gets about 40 spikes over 140°C. About 10 spikes over the limit are acceptable, but not more.

Table 8.3.6 Collector area effect on various system parameters

Collector area (m2) Useful output per m2 Solar fraction Total efficiency Remaining aux collector (kWh/m2a) (%) increase (%) demand per living area (kWh/m2a)

10 260 37 28 30.3

Storage tank size

Another important factor is the size of the tank. Figure 8.3.4 shows the influence on the yearly auxiliary demand for a 7.5 m2 flat-plate solar collector combi-system with increasing tank sizes. The utilization of the solar gains from the solar collector increases with the size of the tank; but, on the other hand, the heat losses also become larger as the size of the tank is increased. The maximum allowed tank temperature is 95°C and the night cool-off temperature was set to 80°C. According to the simulation program, the tank size does not play a central role. An optimum can still be seen at 0.5 m3. Below this threshold the solar gains cannot be fully utilized, and above, the heat loss increase exceeds the increase in solar gains. However, the auxiliary demand does not vary more than 1 per cent as long as the tank size is within a reasonable volume span (0.4-1.0 m3). When the size exceeds this span, the heat losses become quite substantial and, hence, the auxiliary demand increases a great deal. What the figure does not show is that the tank size should not be less than 400 litres in order to be able to cover the DHW and space heating demand. Too small a tank cannot store sufficient enough heat in order to supply warm water when the power requirements are high (i.e. during the winter or when the shower is used for a long time).

36.0

32.0

28.0

20.0

36.0

32.0

28.0

20.0

Source: Tobias Bostrom and Johan Smeds

Figure 8.3.4 Remaining annual auxiliary demand as a function of tank size

Source: Tobias Bostrom and Johan Smeds

Figure 8.3.4 Remaining annual auxiliary demand as a function of tank size

Tilt effect

To increase the solar collector output over the year, the output during spring and autumn must be increased while the output during the summer is suppressed. One way of achieving this is to use concentrating solar collector systems with various acceptance angles; but this is not studied here.

In order to increase the solar fraction during the winter for non-concentrating systems, the tilt angle of the collector has to be raised. The low-standing winter sun can be more efficiently utilized by placing the collector vertically. By having vertical standing collectors, the high-standing summer sun is also suppressed, making it possible to have larger collectors without creating an overheating problem (Bostrom el al, 2003).

Figure 8.3.5 shows how the annual auxiliary demand diminishes as the collector area is increased. Small systems tilted 40° are more effective than the vertical equivalent. However, the 40° tilted system creates a large amount of unusable heat during the summer when the collector area is larger than 10 m2, which results in the vertical system becoming more effective for large areas. Nevertheless, it is not economically justified to double the collector area just to get a 13 per cent decrease in auxiliary demand. It might also be a problem to find a large enough non-shaded south-facing façade for the solar collector. Advantages and disadvantages with either mounting position can be found in the Table 8.3.7.

Table 8.3.7 Pros and cons with a roof- or wall-mounted collector for cold climates 40° tilt 90° tilt (vertical)

Pros Cons Pros Cons

Possible to shut off the auxiliary system during the summer

Small collector areas give a higher solar fraction Generally the more economical choice

Susceptible to overheating during the summer

Becomes covered by snow in the winter

Acquires the highest solar fraction (large systems)

Cheaper to install

Boosted solar radiation through snow reflections during the winter

Needs larger areas

More easily shaded

Smaller available surface area to install the collectors on

Boosted solar radiation through snow reflections during the winter

Evacuated tube versus flat-plate collector

An alternative to using flat-plate collectors is evacuated tubular collectors. Vacuum systems have a higher efficiency compared to flat-plate systems but are, on the other hand, about twice as expensive per m2. To compare, the optimal flat-plate combi-system with 7.5 m2 tilted 40° which needs an auxiliary demand of 31.8 kWh/m2a is matched with the vacuum system that needs the same amount of auxiliary energy. Simulations show that 5 m2 of evacuated collectors tilted 40° are sufficient to achieve the same efficiency. In other words, the evacuated collector is about 50 per ent more effective per area unit than the flat-plate collector; but since the evacuated collector is twice as expensive, it does not become an economically justified choice today. These findings correspond well with an in situ measurement (Kovacs and Pettersson, 2002), which showed that evacuated collectors were between 45 per ent and 60 per cent more effective than flat plates per m2, depending on the load applied. Both the flat plate and the evacuated tube simulated in this chapter are modern high-performance collectors.

Hot water set temperature

The discussion has so far concerned the solar collectors and the storage tank; but the auxiliary system is also important. One very important parameter is the domestic hot water set temperature, TDHW, which is the actual temperature at the faucet. The temperature at the top of the tank is usually a few degrees higher in order to cope with the temperature drop from the tank to the faucet. TDHW has been set to 50°C in the simulations above, which is an adequate temperature for household purposes. Many solar collector systems have a much too high TDHW, quite often up to 70°C or more, which results in the fact that the needed auxiliary energy drastically increases. This is often due to the fear of Legionella disease. However, a temperature of 50°C is sufficient to prevent growth of Legionella. Simulations show that the auxiliary energy demand for a 7.5 m2 flat-plate combi-system tilted 40° decreases with 5% if TDHW is decreased from 60 to 50°C.

DHW versus combi-system

Both systems have 7.5 m2 of flat-plate collectors tilted 40° and a 500 litre tank. The remaining auxiliary demand for the combi-system was 31.7 kWh/m2a (living area); the corresponding figure for the DHW system was 32.2 kWh/m2a. Highly insulated buildings will consequently only benefit to a very small degree from having a conventional solar combi-system; a DHW system will provide about the same amount of useful heat since it is mainly the spring, summer and autumn DHW load that is covered by both systems. A roof-mounted collector will produce very little useful heat during the short heating season of November to March.

There are a number of approaches in order to increase the efficiency of a solar combi-system. As mentioned earlier, the collector can be tilted vertically and thus achieve a higher efficiency for the low-standing winter sun. Alternatively, one might use concentrating systems that can boost the collector performance for specific solar angles. When choosing your heating system, you should also opt for a low temperature heating alternative that allows the collector to work more efficiently (i.e. floor heating).

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