Heating Systems. Passive solar Furnaces Boilers Wood-burning stoves District heating Electric resistance heating Heat pumps On-site cogeneration

Transcription

1 Heating Systems Passive solar Furnaces Boilers Wood-burning stoves District heating Electric resistance heating Heat pumps On-site cogeneration

2 Passive Solar Heating Direct gain Solar collectors Air-flow windows

3 Not all solar gain is usable some leads to overheating, requiring the windows to be opened To maximize the useful solar gain, thermal mass (such as concrete or stone) is needed and should be exposed to the indoor air (so minimize interior finishings) (this is the new look anyway in many buildings now) With thermal mass, absorbed solar energy goes into storing heat with minimal temperature rise (apart from being uncomfortable, high temperatures result in greater radiant and convective heat loss, and thus less heat available for later)

4 At night, the heat is slowly released when there is high thermal mass. This is adequate if the building is highly insulated with high-performance windows. If there is too large a glazing fraction (which typically means > 60%), there will be more solar gain than can be used, and greater heat loss at night

5 Example of fan-assisted passive solar heating in a Japanese school Source: Yoshikawa (1997, CADDET Energy Efficiency Newsletter June, 8 10)

6 Air-flow windows to preheat incoming ventilation air

7 Triple-glazed air flow window serving as a counterflow heat exchanger Source: Gosselin and Chen (2008, Energy and Buildings 40, ,

8 Finnish supply-air window Source:

9 Boilers, Furnaces Non-condensing, 75-85% full-load efficiency, lower efficiency at part load (which is achieved through on/off cycling) Condensing, 88-95% full-load efficiency, greater at part load (which is achieved through modulation of the fuel and air flow) and with lower return temperatures (because more water vapour can be condensed and used to preheat the return water flow)

10 Efficiency of a condensing boiler vs temperature of the water returning to the boiler from the heating loop, and vs load Thermal efficiency (%) % input 50% input 100% input Return water temperature ( o C) Source: Durkin (2006, ASHRAE Journal 48, 7, 51 57)

11 Pellet-burning boilers 86-94% efficiency Have a maximum output as low as 10 kw and can operate between % of maximum output (we want the capability for minimal output in super-insulated houses) Largest units have 40 kw peak output Pneumatic delivery of pellets from trucks to storage bins in houses Automatic transfer of pellets to the burner and removal of ash Common in Austria

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13 Electric Resistance Heating 100% efficiency at the point of use Easily controlled can supply just the amount of heat required and no more In super-insulated houses, about 1/3 of the total heat required comes from waste heat from lighting, appliances and electronic equipment, so a significant fraction of the heating is already electric Overall efficiency including loss at the electric powerplant (which is typically coal fired) and transmission - can be quite low (30-40%) However, if electricity is supplied on the margin by renewable electricity at certain times then, in a superinsulated house, one could use electricity for heating only or mostly at those times and let the temperature drift in between

14 Heat Pumps This is an alternative electric heating system Electricity is used to transfer heat against its will, from cold to warm Typically, 1 unit of electricity can provide 3 units of heat so this nullifies the losses associated with the roughly 33% overall efficiency in supplying electricity from coal plants at the typical 35-40% generation efficiency

15 Heat Pump, Operating Principles Heat pumps transfer of heat from cold to warm (against the macro temperature gradient) At each point in the system, heat flow is from warm to cold Heat pumps rely on the fact that a gas cools when it expands, and is heated when it is compressed, creating local temperature gradients contrary to the macro-gradient

16 Components of a heat pump Compressor Evaporator Condenser

17 Heat pump in heating mode Reject air O (-5 C) Reversing valve High-pressure O refrigerant (60 C) Outdoor coil as evaporator Heated air O (30 C) Fan Outdoor air O (0 C) Compressor Indoor coil as condenser Low-pressure O refrigerant (-10 C) Expansion device Blower Indoor air O (20 C)

18 Heat pump in cooling mode Reject air O (35 C) High-pressure refrigerant (60 C) Reversing valve O Outdoor coil as condenser Cooled air O (15 C) Fan Outdoor air O (30 C) Compressor Indoor coil as evaporator Expansion device Low-pressure O refrigerant (-10 C) Blower Indoor air O (25 C)

19 Heat Pump, Efficiency Principles The ratio of heat delivered to energy input is called the coefficient of performance (COP) The maximum possible COP (called the Carnot cycle COP) is related to the temperature lift, T H -T L, where T H =condenser temp and T L =evaporator temp COP cooling,carnot = T L /(T H -T L ) COP heating,carnot = T L /(T H -T L )+1.0 The actual COP (in the case of cooling) is given by COP cooling, real = η c (T L /(T H -T L )) where η c is the Carnot efficiency

20 Heat Pump COP in heating mode 10 n c = Heating COP 6 4 Condenser Temperature: 30 o C 50 o C 70 o C 90 o C Evaporator Temperature ( o C)

21 Heat Pump COP in Cooling Mode (or chiller COP) 12 n c =0.65 Cooling COP o C -5 o C 0 o C Evaporator Temperature: 5 o C 10 o C Condenser Temperature ( o C)

22 Heat flow, temperature lifts, and COPs of a heat pump in cooling mode

23 Thus, to reduce heat pump energy use, Distribute heat at the lowest possible temperature (e.g., at 30ºC instead of 60ºC using radiant floor or ceiling heating) Distribute coldness at the warmest possible temperature (e.g., at 20ºC instead of 6ºC using chilled ceiling or chilled floor slab) Minimize ΔT H and ΔT L by - minimizing the required heat flows (which must balance heat loss or heat gain, so this means a super-insulated building with high-performance windows) - using as large a radiator surface as possible

24 Sources of heat for a heat pump: The outside air (gives an Air-Source Heat Pump) The ground (gives a Ground-Source Heat Pump, now quite incorrectly called geothermal heating by vendors of this equipment) The exhaust air (gives an Exhaust-Air Heat Pump now standard practice for new houses in Sweden) (extracts more heat from the outgoing exhaust air than a simple heat exchanger)

25 Ground Source Heat Pump, horizontal pipes (a) Foundation Wall Supply Runouts 30 m Return Runouts 84 m 30 m x 84 m available surface area Source: Caneta Research Inc (1995, Commercial/Institutional Ground-Source Heat Pump Engineering Manual, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Atlanta)

26 Ground Source Heat Pump, vertical pipes (b) Foundation Wall Supply Runouts Return Runouts 15 m 46 m 15 m x 46 m available surface area Source: Caneta Research Inc (1995, Commercial/Institutional Ground-Source Heat Pump Engineering Manual, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Atlanta)

27 The ground is a better source of heat than the air because, during the winter, the ground might be at 8-10ºC while the outside air could be at -20ºC. Conversely, during the summer the ground will be cooler than the air and so it is a good heat sink However, if a ground-source heat pump is mostly used for winter heating, the ground will get progressively colder from one year to the next, while if a ground-source heat pump is used mostly for air conditioning, the ground will get progressively warmer over time, in both cases reducing the COP of the heat pump.

28 Solutions: Try to balance winter heating and summer air conditioning loads (by shifting the priorities in the design of the building) Circulate hot water from solar thermal collectors to restore ground temperatures during the summer Cool the ground down during the winter by circulating some fluid (with antifreeze) between the ground and some sort of heat exchanger in the outside air

29 The downside of heat pumps is that they have a high upfront cost, although they often pay for themselves over their lifespan A key economic issue will be the ratio of peak heating requirement to average heating requirement (a lower ratio will be more favourable). This will be affected by the character of the envelope, building thermal mass, and the building surface/volume ratio (which is smaller in multi-unit than in single unit residential buildings)

30 If a building has a high-performance envelope (so that heat is lost or gained slowly) and a high thermal mass (so that the temperature change for a given heat loss is small), then the heating or cooling system can be turned off for some period of time without an important effect on the building temperature. Thus, if the heating and cooling are provided by electric heat pumps, then we have an electric load that can be ramped up or down to match variations in the supply of C-free electricity. If we are running a heat pump when, example, there is excess wind-derived electricity supplied to the grid, and not running it at other times, we are in effect using the building thermal mass to store wind energy in the form of useful heat (or coldness during the summer season when the heat pump is used as an air conditioner).

31 In summary, a high-performance envelope saves fossil fuel energy in 3 ways By reducing the heating load (the amount of heat that needs to be provided) By increasing the efficiency of a furnace, boiler or (especially) of a heat pump in providing the required heat By providing flexibility as to when heat is provided (this flexibility is amplified if the building has a high thermal mass)

32 REDUCING COOLING ENERGY USE Reduce the amount of heat that a building receives, thereby reducing the cooling load (the amount of the heat that needs to be removed) Use passive and low-energy techniques to meet as much of the cooling load as possible Use efficient equipment and systems to meet the remaining cooling load

33 Cooling load in a Los Angeles office building Roof 8% Walls 3% Fresh Air 10% Lighting 28% Windows 21% Fans 13% Office Equipment 5% People 12%

34 Cooling load in a typical Hong Kong building Roof 0% Walls 4% Fresh Air 20% Lighting 18% Fans 10% Windows 8% Office Equipment 13% People 27%

35 Reducing Cooling Loads Building orientation and clustering High-reflectivity building materials External insulation External shading devices Windows with low SHGC Thermal mass Vegetation (provides shading and evaporative cooling) Efficient equipment and lighting to reduce internal heat gains

36 Thermal Mass By itself, does not reduce the cooling load High thermal mass means that it takes longer for the building to warm up, but with a prolonged heat wave, a building with high thermal mass eventually heats up (and then will take a long time to cool down) However, thermal mass will greatly reduce the temperature increase from morning to late afternoon, so if the night becomes cool enough, night air can be used to remove heat from the thermal mass so that it does not build up from day to day (or at least not as much)

37 To most effective, thermal mass needs to be combined with External insulation Night-time ventilation with cool outside air flowing into the core of the thermal mass (such as hollow concrete slab ceilings or walls) In effect, the coldness of the night air is stored and used to keep the building cool during the day This of course reduces total energy use but also reduces required peak rates of mechanical cooling saving on purchase costs for cooling equipment and electrical transformers, and reducing utility charges to meet peak electricity demand

38 The traditional materials used to add thermal mass are concrete and stone However, phase change materials can also be used either as small spheres in regular plaster or in the ventilation air flow. These are waxes that can be designed to melt at, say, 26ºC, absorbing heat in the process and resisting any further increase in air temperature. If the air temperature drops below 26ºC at night, they will refreeze (releasing heat that is taken away with the night-time air flow), ready to absorb heat again the next day. These would be ideal in arid parts of the world (where nights get cold and days are hot)

39 Micro-encapsulated phase-change material (left) and spheres containing phase change materials in an air flow pipe (right) Source: Schossig et al (2005, Solar Energy Materials and Solar Cells 89, , & Arkar and Medved (2007, Solar Energy 81, ,

40 Double skin facades Permit adjustable external shading on tall buildings Permit day and night ventilation when it would not otherwise be possible In so-doing, they can greatly reduce cooling loads Design details are important, however

41 Comparison of double-skin façades (DSF) and single-skin façades (SSF) with moderate or high levels of insulation and normal or optimal ventilation strategies with regard to heating load in a 5-story office building in Belgium Heating Load (kwh/yr) Moderate, SSF Base Moderate, SSF Opt Moderate, DSF Opt High, SSF Base High, SSF Opt High, DSF Opt

42 Comparison of double-skin façades (DSF) and single-skin façades(ssf) with moderate or high levels of insulation and normal or optimal ventilation strategies with regard to heating load in a 5-story office building in Belgium cooling load in a 5-story office building in Belgium Cooling Load (kwh/yr) Moderate, SSF Base Moderate, SSF Opt Moderate, DSF Opt High, SSF Base High, SSF Opt High, DSF Opt

43 Lessons on DSFs from the previous slides The insulation level is far more important than adding a second skin for the heating load The building operating strategy (opening windows when appropriate, and appropriate use of day and night-time ventilation) is far more important than adding a DSF for the cooling load If there is already a sensible operating strategy, adding a second facade can increase the cooling load However, the second facade may be necessary to permit a sensible operating strategy in the first place (by protecting against wind, noise, dust and intruders (human or animal) with open windows) Based on simulations for a 5-story office building in Belgium, the combination of modestly higher insulation levels and modestly better glazing with addition of a second facade and the use of the natural ventilation that it permits reduces heating energy use by ~50% and cooling energy use by ~80%