Row house in the Cold Climate Renewable Energy Strategy

Joachim Morhenne

Table 8.5.1 Row house targets in the Cold Climate Renewable Energy Strategy

Targets

Space heating

20 kWh/m2a

Non-renewable primary energy:

(space heating + water heating + electricity for mechanical systems)

60 kWh/m2a

This section presents a renewable energy solution for the row houses in the cold climate. As a reference for the cold climate, the city of Stockholm is used.

8.5.1 Solution 2: Solar domestic hot water and solar-assisted heating

The use of a solar combi-system and efficient mechanical ventilation with heat recovery are essential to achieve the space heating target of 20 kWh/m2a (2400 kWh/a per unit) in this climate for row houses. Applying these two measures then allows some freedom as to how much transmission losses must be reduced and still meet the target. These two measures, together with the good A/V ratio of row houses, mean that the target can be met with an envelope construction only slightly better than current building code requirements. Consequently, this strategy is also applicable to building retrofit where improving the building envelope may be difficult.

To achieve the primary energy target, a solar domestic water system with a 60 per cent solar fraction is adequate. A larger solar combi-system will therefore reduce the primary energy demand well below the target of 60 kWh/m2a.

Why follow this strategy?

Solar gains make it possible to reach the target without having to apply excessive conservation measures. Given that most high-performance houses today have a solar domestic hot water system, this strategy proposes to increase the solar system to also provide some space heating. As a result, the target can be met without, for example, using expensive high-performance windows.

Because this solution uses surface and not ventilation air to deliver the needed heat, the heat delivery capacity is not limited by the ventilation rate. Furthermore, surface heating provides superior comfort. Finally, this strategy can compensate for low passive gains if the building is not optimally orientated or is shaded.

Building envelope

Opaque construction: massive or lightweight walls with exterior insulation. Thermal mass increases summer comfort and slightly increases the usefulness of solar gains. In case of lightweight walls, floors and ceilings should at least be massive.

Windows: frame ratio of 30 per cent, double low-e coated glass with argon gas.

Table 8.5.2 Building envelope U-values

Component U-Value (W/m2K)

Floor 0.21

Walls 0.20

Walls east/west (end houses) 0.16

Roof 0.16

Window glass 1.2

Window frame 1.7

Mechanical systems

Ventilation: mechanical ventilation with 80 per cent heat recovery. Ventilation rate: 0.45 ach. Infiltration rate: 0.05 ach. Electric consumption: 0.3 W/m2h.

Heat supply: central condensing gas furnace, biomass boiler or connection to local grid. Four-pipe heating grid for each row of houses.

Solar system: central solar combi-system or individual solar combi-systems for each unit. Heat distribution: hot water floor or wall radiant heating, supply air heating.

Figure 8.5.1 explains the solar heating system. The solar system is a typical solar combi-system with six houses each with 10 m2 collector area. Important questions are whether the solar system is private or common property and how investment and maintenance costs are shared. In contrast to collective systems, individual systems need more space for the storage, investment costs are higher and surplus heat when one family is away cannot be shared. However, there are no grid heat losses.

Solar heat is used to raise the temperature of the return flow from the floor heating (see section 8.5.3). For further information about solar combi-systems, see the final report of the IEA-SHC Task 26 (Weiss, 2003).

Figure 8.5.1 explains the solar heating system. The solar system is a typical solar combi-system with six houses each with 10 m2 collector area. Important questions are whether the solar system is private or common property and how investment and maintenance costs are shared. In contrast to collective systems, individual systems need more space for the storage, investment costs are higher and surplus heat when one family is away cannot be shared. However, there are no grid heat losses.

Solar heat is used to raise the temperature of the return flow from the floor heating (see section 8.5.3). For further information about solar combi-systems, see the final report of the IEA-SHC Task 26 (Weiss, 2003).

Source: Joachim Morhenne

Figure 8.5.1 Scheme of the solar assisted heating system: (a) central system for a row of houses and (b) individual solution

Energy performance

Space heating demand: the monthly space heating demand of the row houses was computed with TRNSYS and is shown in Figure 8.5.2. The row consists of two end houses and four mid houses. Results in Table 8.5.3 are mean values for the row of houses. The heating season extends from 1 October to 31 May.

Without the solar system the net energy demand for heating the building is 21.7 kWh/m2a. In reality, the heating demand totals 26.6 kWh/m2a because:

• the indoor temperature often exceeds 20°C; and

• the grid also loses heat before reaching the house.

Table 8.5.3 shows the performance of the building, including the savings of the solar system. The delivered energy to cover heat demand includes losses of the control system, storage and pipes, as well as from the collector and its circuit. The system losses include the combustion losses of a condensing gas furnace for heating and hot water preparation.

Table 8.5.3 Performance of the building, including the system

Delivered energy for space heating (mean) 19.3 kWh/m2a (2300 kWh/a)

System losses 1.5 kWh/m2a

Solar contribution to space heating demand 16%

Heating set point 20°C

Peak load for space heating: the peak load is 2350 W for the end units and 1800 W for the mid units. While the peak occurs in January by an ambient temperature of -18.9°C, near peak demands also occur in February and December. Outside of these three months, the peaks fall off very sharply.

Delivered energy for DHW: the energy demand for DHW is 1360 kWh/a or 11.3 kWh/m2a. The solar contribution to the delivered energy for DHW is 62 per cent.

Source: Joachim Morhenne

Figure 8.5.3 Energy balance of the reference and solar base case (columns 1 and 3 are gains, columns 2 and 4 are losses)

Delivered energy use: the total end energy use for DHW and space heating is 3850 kWh/a. The electric consumption by fans, pumps and controls is 670 kWh/a. Figure 8.5.3 explains the energy balance for the reference case and the high-performance case.

Primary energy demand and CO2 emissions: the primary energy demand and the CO2 emissions are shown in Table 8.5.4. Factors are taken from GEMIS (2004). The primary energy factors are:

The CO2 emission factors are:

The primary energy demand is 49.8 kWh/m2a and the corresponding CO2 emissions are 10.3 kg/m2a when using gas as a heating source for the remaining energy. If, instead, biomass is used as fuel the primary energy demand is only 15 kWh/m2a and the CO2 emissions are 3.6 kg/m2a.

Table 8.5.4 Total energy use, non-renewable primary energy demand and CO2 emissions

Net Energy (kWh/m2a)

Total Energy Use (kWh/m2a)

Delivered energy (kWh/m2a)

Non renewable primary energy factor (-)

Non renewable primary energy (kWh/m2a)

CO 2 factor (kg/kWh)

equivalent emissions (kg/m2a)

Energy use

Energy source

Mechanical 5.6 systems

Mechanical 5.6 systems

EleC" 5 6 tricity

EleC" 5 6 tricity

2.4

13.2

0.43

2.4

Space 21 ? heating

Space 21 ? heating

Gas 32.1

Gas 32.1

1.1

36.6

0.25

7.9

DHW 30.0

DHW 30.0

Tank, circulation and 59

conversion losses

Solar 25.5

Total 57.3

63.2

63.2

37.7

49.8

10.3

8.5.2 Summer comfort

The heating system is not active in summer; therefore, only internal and passive solar gains and the ventilation air contribute to overheating. The indoor temperature never exceeds 26°C during the simulated year (see Figure 8.5.4). Due to the chosen shading and ventilation strategy, summer comfort is achieved. Better shading devices and increased night ventilation could further improve comfort.

Source: Joachim Morhenne

Figure 8.5.4 Number of hours of the indoor temperature distribution

To reduce electric consumption, night cooling with ambient air by opening windows could be used instead of a mechanical ventilation system. If the ventilation system is used during the summer, an automatic bypass of the heat exchanger is recommended. Note that in these very air-tight houses, it is important to ventilate since the infiltration rate is extremely low.

8.5.3 Sensitivity analysis

System design

The evaluated system is shown in Figure 8.5.5. Due to the cold climate, the performance of the solar system in winter is very sensitive to the return flow temperature of the heating system (see Figure 8.5.7). Therefore a four-pipe grid was chosen to be able to operate the grid with different tempera-

Source: Joachim Morhenne

Figure 8.5.5 Scheme of the central system

Source: Joachim Morhenne

Figure 8.5.5 Scheme of the central system tures for space heating and DHW. The space heating grid has, in this case, a lower supply and return temperature then the hot water grid. This results in a better use of solar gains. Compared to two-pipe grids, they have higher grid losses; but the grid losses can partly be recovered if the grid is installed inside the building envelope.

The most important parameters of the heating system are shown in Table 8.5.5.

Table 8.5.5 Important system parameters

Temperate climate; solar strategy

10 m2 Flat plate

54° 12 l/m2 Maximum efficiency 92% 45 l/m2 South 0.5

Heavyweight

Design temperature Heated surface/heating power Collector area:

base high-performance retrofit

Collector type

Collector slope, south

Flow rate

Control

Heat exchanger Storage Main façade Shading coefficient Construction

Collector area

The influence of collector slope and azimuth angle can be taken from standard tables (see Duffie and Beckman, 1991). The optimum values (azimuth south, slope 54°) are used here. Figure 8.5.6 shows the influence of the collector area on the usable collector gains. The usable collector gains are 255 kWh/m2a (per m2 collector) for the collector area of 10 m2. The specific collector gain for heating is about 44 kWh/m2a and cannot be increased much (see Figure 8.5.7). A further reduction of the total energy demand by increased solar gains is only possible by increasing the solar fraction for DHW

The design temperature for the heating system is very important for the operation of the solar combi-system. The useful solar contribution drops drastically as the return flow temperature increases. For this reason, only low temperature floor or wall heating systems are recommended.

Collector (m2)

Collector (m2)

Source: Joachim Morhenne

Figure 8.5.7 Influence of the collector size on usable solar gains for a reduction in heating demand depending upon the supply temperature of the heating system

Primary energy demand depending upon collector area: Figure 8.5.8 shows the primary energy demand depending on the collector area when natural gas supplies the remaining needed energy.

Influence of the supply temperature of the heating system : the temperature of the supply and return flows of the heating system has a high influence since the solar system is always connected to the return flow of the system. The influence of supply temperatures is approximately 8 per cent to 10 per cent, as can be seen in Figure 8.5.7.

Influence of the storage size: the storage volume of the solar system does not strongly affect system performance unless the storage size exceeds a critical value. Within the range of 42 to 57 l/m2 of collector the influence of the storage is less than 3 per cent.

Influence of the flow rate in the collector circuit: since the solar collector heat is transferred to the return water flow for space heating, logically, the collector outlet temperature must exceed the return flow temperature for heat to be transferred. Therefore, the flow rate has to be reduced until the necessary outlet of the collector is reached independently of the collector efficiency. A dynamic control of the flow rate of the collector loop optimizes system performance. In the simulations, 12 l/m2h have been used. Other parameters, such as the thermal mass of the building, have been analysed for the temperate climate.

Getting Started With Solar

Getting Started With Solar

Do we really want the one thing that gives us its resources unconditionally to suffer even more than it is suffering now? Nature, is a part of our being from the earliest human days. We respect Nature and it gives us its bounty, but in the recent past greedy money hungry corporations have made us all so destructive, so wasteful.

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